Yes there should be, it is very satisfactory to have complete control over your defence and attack rather than relying on 7 random team mates who will probably quit after they complete their daily challenge to send 20 troops or they place random towers because they don’t know how to play.
Games like Bloons Tower Defence or other tower defence games are pretty much always 1v1s rather than team vs team.
Also some new players who don’t know how to play well yet just want to experiment and see what tactics work etc but if they do something wrong the rest of the team will start abusing them or report them for “trolling” so a 1v1 version of tower defence could be a place they can practice without breaking rules or making people mad too.