Capacity and BER as performance of transmission system
Channel Capacity and Bit/Symbol Error rate are two different key performance indicators (KPI) of a system.
BER/SER, as it's obvious from its name, is  the number of Bit/Symbol errors divided by the total number of transferred Bits/Symbols. and you can use channel coding to improve it so it's not a fixed KPI.
while Channel Capacity is maximum rate that the system can achieve and it's a fixed KPI.
so it make sense to study the channel capacity of your system. by studying channel capacity you find maximum rate that can be achieved and by improving BER/SER you can improve rate of system to get close to channel capacity.