Ancestry isn't the only option though, is it?
Of course it isn't, but you referred to Ancestry in your question. I assume you have tested with Ancestry, or you wouldn't have asked the question that you did. You can transfer your test data to several other providers such as My heritage, LivingDNA/FindMyPast, Family Tree DNA, GEDmatch etc. to obtain matches with their databases, but if you haven't tested with Ancestry you can't transfer in to them.
But you asked whether it was necessary to have a tree on Ancestry. If you haven't tested with Ancestry and you aren't in their DNA database, uploading or creating a tree is of no value whatsoever for DNA purposes. If you have tested with Ancestry, which your previous question intimated, a tree is still not essential; but without it you will not get any Thrulines suggestions to your DNA Matches, which could save you many hours of investigative research in establishing how you and some of your matches may be related.
My Heritage have a similar feature. Again, it's not necessary to have a tree on My Heritage, but their smart matches won't give you hints without one.
GEDmatch strongly suggest that you upload a Gedcom if you transfer in to them. Again, it's not necessary, but in each of these examples you are not taking full advantage of the resources available to you if you don't.
Personally, if I'm looking for or investigating DNA Matches, I want to be able to take advantage of every tool available to me that might help my research. I see no point it making things harder or more difficult than necessary.
As I said, my Ancestry tree (and those on other sites such as My heritage and FindMyPast) are bare Gedcoms, and private. No one else can see them unless they ask me for permission, but they work for hints and Thrulines/Smart Matching, which is why I have them there.