the secondary interface will be use to communicate using another IP subnet but with oruting wil not be considered as the rotuing will consider the main primary interface IP

can be used with static route

subinterface is as you have difrent interface the given Ip will be considered as primary IP at that subinterface

also if you have an ethernet interface connected to trunk you can you the subinterfaces as each one i diffrent VLAN this helps in tinter vlan routing also known as router-on-stick

with frame-relay subinterfaces can be put each in diffrent DLCI so they operate thorugh diffrent circuit

if helps Rate