I and my father have both had autosomal tests at 23 and me. Initially I did mine because I'd done some courses in genetics and developed an interest. I didn't even consider it's use in genealogy
I later had my father tested because this would enable me to see whether people I matched DNA with, matched on my mothers or on my father's side. It narrows the field considerably.
I have lot's of matches on both sides but they are only of any use when the other person also has a family tree; then just occasionally you may be able join the dots and make a connection . I recently found a match on my mother's x chromosome with a person in Canada. It transpired that we shared the same 5xg grandmother. He hadn't got back that far and I hadn't traced this sibling line across the Atlantic.
Ethnicity is a different matter though. It's mildly interesting but no more than that. My ancestry is firmly rooted in the Midlands with all 16 gg grandparents born there (12 of them from Northamptonshire). I think my results are exactly the sort of mix I would expect in central England .
https://www.theguardian.com/science/2015/mar/18/genetic-study-30-percent-white-british-dna-german-ancestry 50.0% From Mother (she has all of g grandparents from Northamptonshire)
23.9%British & Irish
6.7%French & German
3.1%Scandinavian
16.1% Broadly Northwestern European
0.1% Ashkenazi
0.2% Broadly European
50.0%From Father (4g grandparents from NTH, the rest from the West Midlands, a couple of whom might have recent ancestors elsewhere)
34.7% British & Irish
4.6% French & German
0.4% Scandinavian
9.3% Broadly Northwestern European
0.1% Ashkenazi
0.7% Broadly European
< 0.1% Middle Eastern & North African
< 0.1% Broadly Middle Eastern & North African
0.3%Unassigned