Why should i use hair cuticle sealer?

Hair is naturally porous (has holes in it) and because of how the hair is built (with proteins and such), when it gets chemically treated (dyes, bleach, perms, straightener treatments) it loses some of its proteins. This means your hair has more gaps and is now weaker.

When you dye your hair though, the colour seeps into each, individual hair that it is applied to. This does not actually fill the holes though. So that means after your hair has been coloured and dried and such, the colour can leak out quickly if your hair has become too weak/thin. There are two things a person does to combat this.

Protein filler/pre treatment - this is usually applied to the hair right before dyeing it, or it is mixed in with the hair dye (particularly if one is using professional dyes and developers). This helps keep the hair stronger so that the colour is accepted and kept.

Cuticle sealer - the process of treating your hair opens up the hair sort of (think of a pinecone, when the seeds have not popped out of it, a pinecone has flat sides, but then it opens up, and then it has sharp open areas), and this can cause your hair to have more breakage, and to cause your hair colour to seep out very quickly. Cuticle sealer is applied after the dye has been rinsed out of the hair.

So you should use cuticle sealer to protect the colour and to protect the hair.