Both up- and downvotes do have a significant function. I actually prefer the lemmy method where they are presented separately.
A simple example could be a post such as:
YSK: The world actually is flat
-contains link to some bogus videos and stuff
In a lemmy w/o downvotes, it could show as only (10 upvotes) until a mod gets around to removing it, whilst in one with downvotes this post would quickly accrue downvotes if it gained any amount of traction.
I used a simple example here, but my point is that the combination of up and downvotes provide an easy gauge of general sentiment towards the statement/comment/post.