The idea seems to be to make connections with possible family members, however I think it may have another useful purpose - identifying those who are NOT connected to your tree.
For example, I can see that one particular user has added two of my ALLEN census records to their own tree. One record is for William H Allen but this user has added it to a William G Allen in their tree. The same user has added an 1861 certificate to another person who I do not recognise as a member of this branch.
It is possible that this user has added these records to their tree in error - probably misreading the handwriting (Who hasn't done that?). With this feature, we can make contact quickly and establish whether a mistake has been made so the user does not have to waste any further time, money and energy.
I must confess that I've let my ancestry membership lapse so I can't actually check these tree 'connections'. They may very well be true connections or it is possible that I am the one who is incorrect. Nevertheless, I think it is extremely helpful to find out sooner rather than later that you are barking up 'the wrong tree'.