Using different registrars for different domain names and for different purposes can be good for many reasons.
First off, security. If someone gains access to your account at a registrar and all your domains are in that same account then there is potential to loose them all. Even with 2 factor login, it can and DOES happen more often than you would think. Having your names split between different accounts at one registrar or among different registrars can limit loses if that does happen.
Another reason that people might use multiple registrars is that not all registrars sell all the many hundreds of domain extensions that are available to register. This is another reason to use multiple registrars.
one more reason is price. There can be a big difference between registration and renewal prices of domains between the many registrars so shopping around is practical for some people.
Even with the many reason to have and use multiple registrars, I would recommend looking into any registrar and their reputation before choosing to go with them. There are a few that have some really cheap domains, yet have reputations as a bad registrar or a PITA to work with. Some even have some shady business practices. So do some research before you register a domain with any registrar.
I personally have 5 different that I use, all for a combination of the reasons above. I have used a couple dozen over about a 20 year period and have had both good and bad experiences. I have to say that in the early years it was a lot easier deciding on which registrar to use since there weren't a countless number of them like there are now!