Once you've answered that, you've got yourself a USP.
Or do I misunderstand the concept?
Trust me there's no window cleaner I know of that has a USP.
In your 2 w/c example, the answer is "chemistry".
We all (like it or not) decide within seconds of meeting somebody for the first time whether we like them or not.
In this case, the USP would be LQ = likeablity quotient and, yes, it's important.
(It also works for customers. I had a phone call today from a lady who wanted a conservatory cleaned. I went round and had a look at the job. Didn't like it. Took a long look at the lady. Didn't like her, LQ = -8. Declined the job.)
Trust me, there is no window cleaner that DOESN'T have a USP. The only questions are (i) do you know what it is and (ii) can you utilise it to get more business.
Repeat, ask yourself why, when all other things are equal, or nearly so, why should someone choose you over anyone else.
I decided that highlighting my ability to clean cottages was an advantage. Cottage owners seem to agree => USP.
A USP could be something like:
* guaranteed next day clean
* cheaper than anyone else
* clean the frames and the panes
* fastest window cleaner in the west
* fast, friendly and efficient service.
* free clean insides every fifth clean.
You can find lots of them if you look.
If you've got no competition or you don't care about prices, you don't really need a USP.
If you've got competition or you want to charge above-average prices, you need one.