Mostly around 2012 to 2015 you will see a lot of android phones being rooted to further customize or improve their phones by adding their own personalized theme, removing bloatwares that comes installed in a phone, and also overclocking and undercloking, all of these things I have said can be done now by a unrooted phone.
I just want to add a little bit. Besides customizing or improving, people root their phones in order to install a proper ROM for use. Back in the days, many good phones are made for domestic markets only or not distributed largely (Japanese and Chinese manufaturers are pretty well known for this). Nearby countries' users could buy their phones however they didn't support their local languages. Hence, rooting was the solution.
Nowadays, every manufaturer produces both domestic and international versions of their phones. Rooting for ease of seems becomes unnecessary.
Generally, I think rooting brings more harm than goodness, even with pro users: viruses and malwares can be featured in any app on CH Play, while mobile anti-virus apps have never been effective.