Hi OP, depends what you want to get out of the testing, and there are various ones.
Majority, including Ancestry are autosomal and will test DNA you inherit from both parents. There are also tests that do Y DNA (men only) and mitochondrial DNA which is your direct female line and can show you the ancestry of your female lineage.
The ethnicity estimates are really a bit of fun. Each testing company uses different algorithms and groups of people as their subjects for an area. They also change these with updates. However if you have say 1% Spanish pop up, you might not be able to verify this on your paper trail tree. A lot of people who come from the British Isles show up with Scandinavian roots (bloody Vikings)
The real benefit to DNA testing are the databases of all of the others who have tested and show up as related to you. Majority of cousins I have matched with are 4th cousins and I have traced how we are related, its like solving a giant logic puzzle. However, as can be reported in the news, beware as sometimes a very close cousin match (or closer!) And then you realise someone within the family had a secret child thus producing relatives that you may never have known about otherwise.
The other DNA tests are health tests and companies can test you for inherited diseases or risks of those, they do advertise which ones. I haven't done this so cannot comment.
Personally as a hobby genealogist having my DNA tested has been invaluable, I also asked my mum to do hers and this helped sort my maternal and paternal matches. So if you are interested in setting up a family tree and getting going, go for it!