Edit: having the domain scriptable / accessible via the AWS API and CLI is a nice feature as well.
In the past I had some domains with my hosting company, some with iwantmyname.com, and a couple with a local registrar (because buying Finnish domains (.fi) requires a personal ID, and most registrars didn't support them).
I started getting annoyed that they were spread amongst different registrars, and I couldn't look in one place to see when things were due to expire. So I picked Amazon as my registrar and moved everything there. (The good news is now they support .FI domains too!)
The biggest benefit though is FREE domain privacy (usually an extra $10-14/year). Also 2 factor auth, and other features... I can't say enough good things about them.
That they only do domains is another big factor for me.
I just wish they supported .au domains
They then "gave in" and said "OK, we can do this manually, on request, for $500 per-request".
Never again.
Some registrars are only good for initial registration, as renewal rates are out of line. Namecheap also has reasonable WhoIsGuard. Basically they run a business based on the value they provide.
I know buying a domain is not a complex thing nowadays at most registrars, but it was one of the best/smoothest webshop experience I ever had.
But Google domains win for the free privacy guard (GoDaddy charges $25/yr for domain & privacy, Google domains is $12/yr)