Is there a reason to create more than one Sector in Stellaris?

The impossibilty to add a colony to a sector due to distance is the main reason at the moment.

Additionaly: You can specialize sectors and set taxes.

You might want to create a sector specialized in research, another in finance, another in mineral production etc.

You mgiht also want to create a builder sector, where new colonies land, which has lots of resources and no taxes to allow for quick growth and then switch the system to another sector.

As someone already mentioned in a comment, another reason is to more easily manage the factions/reduce the potential damage of a sector breaking away.

Having a fairly large sector with important planets break away can totally ruin your empire. Having multiple smaller sectors mitigates that risk

A third reason, unless the UI is faking me out, is that each sector has an energy/mining cap (which in my last game at least was equal to my main cap). If you're running a surplus of energy credits and/or mining credits (and can spare the influence to get 75% of it later), you can create a 2nd or 3rd sector and mainly just use it as a dumping ground for resources until you need to cash it in. (This assumes you don't see anything e.g. in How should I spend energy credits? that you'd rather use your excess for)