we have 3 external web urls which will come on our frontend f5 and redirect to internal web urls and use as a revers proxy

I will be incorporating a client SSL profile suppose if i will use port 443 on VIP still HTTP profile required?

Yes, you'll still need and HTTP profile on the VIP to be able to use the above iRule.

could you please explain why you used default {HTT::host "site1in.com" } at the end.

It was a guess. It's always good to include a "default" condition in a switch, so I guessed at what you'd want if the host name didn't match any of the previous tests.