It was more or less a subline group gimmick, and yeah, it was designed to sell more figures. After all, you need at least two figures to make the combination.
However, I honestly think it's a really fun gimmick
. Yes, some of the combinations are questionable, most of them have giant fender hands, posebility varies greatly between combos, and in the show it really didn't seem to matter one lick. But, despite of all that, it's fun transforming and combining them, and since all but Towline(sorta) could form either a "shirt" or "pants", it really added to the playability of them. Also, you could mess around and combine the weapons, so that was cool.
Fun fact: I'm 90% sure the Energon Powerlinkers were homaging the G1 Multiforce from Victory:http://tfwiki.net/wiki/Multiforce
The only thing missing was the ability for everyone to combine into a giant gestalt.