The external port was a requirement for kits like this

https://images-na.ssl-images-amazon.com/images/I/411PQXERG6L...

But signal testing kits also used it.

This survived until 2G the 5110 still had it.

When 3G came out the coverage was good enough and I’m guessing that issues with signal strength in cars was more or less solved.

What a blast from the past... I remember installing that exact kit (plus antenna) in my own car some twenty years ago. It worked great.