Why did Abram in Genesis 13 build an altar?

Abraham was not the only one. For example, Noah offered a thank-offering to God on leaving the Ark after the Flood:-

Gen 8:20-21

And Noah builded an altar unto the LORD; and took of every clean beast, and of every clean fowl, and offered burnt offerings on the altar.

And the LORD smelled a sweet savour;....

(God likes the smell of barbecues.)

Sacrifices were commanded to be done at particular times and seasons:-

Num 28:2 Command the children of Israel, and say unto them, My offering, and my bread for my sacrifices made by fire, for a sweet savour unto me, shall ye observe to offer unto mein their due season.

Sacrifices could also be done any time. There were sacrifices for sin, but there were also sacrifices for offering thanks to God. In this case, God has promised Abraham, (a wandering nomad with barren wife and hence no children) much land for perpetuity and also many descendants: Abraham was grateful, and voluntarily giving a thank-offering to God:-

Gen 13:14 And the LORD said unto Abram, after that Lot was separated from him, Lift up now thine eyes, and look from the place where thou art northward, and southward, and eastward, and westward:

Gen 13:15 For all the land which thou seest, to thee will I give it, and to thy seed for ever.

Gen 13:16 And I will make thy seed as the dust of the earth: so that if a man can number the dust of the earth, then shall thy seed also be numbered.

Gen 13:17 Arise, walk through the landin the length of it and in the breadth of it; for I will give it unto thee.

Gen 13:18 Then Abram removed his tent, and came and dwelt in the plain of Mamre, which is in Hebron, and built there an altar unto the LORD.

Abraham built another alter :-

Gen 22:1 And it came to pass after these things, that God did tempt [ Note:-If you fail a test, you are tempted, but if you pass it you have been tested.] Abraham, and said unto him, Abraham: and he said, Behold, here I am.

Gen 22:2 And he said, Take now thy son, thine only son Isaac, whom thou lovest, and get thee into the land of Moriah; and offer him there for a burnt offering upon one of the mountains which I will tell thee of.

Gen 22:3 And Abraham rose up early in the morning, and saddled his ass, and took two of his young men with him, and Isaac his son, and clave the wood for the burnt offering, and rose up, and went unto the place of which God had told him.

Gen 22:4 Then on the third day Abraham lifted up his eyes, and saw the place afar off.

Gen 22:5 And Abraham said unto his young men, Abide ye here with the ass; and I and the lad will go yonder and worship,and come again to you.

Gen 22:6 And Abraham took the wood of the burnt offering, and laid it upon Isaac his son; and he took the fire in his hand, and a knife; and they went both of them together.

Gen 22:7 And Isaac spake unto Abraham his father, and said, My father: and he said, Here am I, my son. And he said, Behold the fire and the wood: but where is the lamb for a burnt offering?

Gen 22:8 And Abraham said, My son, God will provide himself a lamb for a burnt offering: so they went both of them together.

Gen 22:9 And they came to the place which God had told him of; and Abraham built an altar there, and laid the wood in order, and bound Isaac his son, and laid him on the altar upon the wood.

Gen 22:10 And Abraham stretched forth his hand, and took the knife to slay his son.

Gen 22:11 And the angel of the LORD called unto him out of heaven, and said, Abraham, Abraham: and he said, Here am I.

Gen 22:12 And he said, Lay not thine hand upon the lad, neither do thou any thing unto him: for now I know that thou fearest God, seeing thou hast not withheld thy son, thine only son from me.

Gen 22:13 And Abraham lifted up his eyes, and looked, and behold behind him a ram caught in a thicket by his horns: and Abraham went and took the ram, and offered him up for a burnt offering in the stead of his son.

Gen 22:14 And Abraham called the name of that place Jehovahjireh: as it is said to this day, In the mount of the LORD it shall be seen.

Gen 22:15 And the angel of the LORD called unto Abraham out of heaven the second time,

Gen 22:16 And said, By myself have I sworn, saith the LORD, for because thou hast done this thing, and hast not withheld thy son, thine only son:

Gen 22:17 That in blessing I will bless thee, and in multiplying I will multiply thy seed as the stars of the heaven, and as the sand which is upon the sea shore; and thy seed shall possess the gate of his enemies;

Gen 22:18 And in thy seed shall all the nations of the earth be blessed; because thou hast obeyed my voice.

Abraham offered this sacrifice in faith because he knewthat either Isaac would be resurrected if killed as a sacrifice, or a substitute would be found. God had promised that Abraham would have many descendants, but since Isaac couldn't do that if he was dead, Abraham knew that even if Isaac died that he had to live again, since God keeps His promises. That can be seen in verse 5, where Abraham tells his servants that they would both return. In any event, this was the forerunner of the future substitutionary sacrifice of Jesus Christ where He took on our sins , and His sacrifice was pleasing to God. Abraham was acting out prophecy, and he knew it.