Why a 5amp. rating ammeter is connected to secondary side of current transformer?

The maximum current output of a CT is 5 amps. Connected to a meter this will be full scale. What ever the ratio of the CT will dictate the scale face on the meter. A 500 amp CT's output will be 5 amps at 500 amps on the buss. 250 amps on the buss, the CT will output 2.5 amp or half scale on the meter face.