Basically, if things seem okay for a while and then later go wrong, the customer might want to switch (both move the domain to a new registrar and change hosting companies). The issue is that your registrar (in this case your host) can make that take about as long as they want (1&1 used to be absolutely awful about this for example). So if you’re taking your hosting business (their bread and butter) elsewhere, they have no reason to make the process an easy one for you. Depending on their own terms, they may even refuse to let you transfer it away (another issue 1&1 used to be an example of).

So it’s always — always, always, always! — a better idea to keep your hosting and domain registrar separate. Just because they seem like a good company now doesn’t mean there won’t be problems (big ones have shut down in recent years causing all kinds of nightmares for customers).

It would be easier to change registrars than a host (where all of your files have to be moved). So if you do opt to split them up, just transfer the domain name away to another registrar. You can do that when it’s renewal time. Many registrars give you a year’s free renewal if you pay their transfer fee (which can even be a dollar or two less than the renewal would have been).