Edberg stated he always had a good kick serve so he used it. He felt it was boring to hit groundies all day and wanted to finish the point at the net. He developed it to perfectly suit his S&V style. He was one of the most consistently fast players to the net behind serve. By the time he recovered after a serve and took his first step to the net he was so far inside the court already.
His older videos (85~86) show his first serve used to be a lot flatter and his second serve tended to have more twist. I don't think the transition into the net was as smooth as in his later years. Lendl at this time commented that Edberg's serve is unlike anything he has faced (referring to good kick and funky body spin according to Mary Carillo). I cannot confirm but somebody stated that Edbeg was emulating McEnroe's serve somewhat (his set up). There is certainly some resemblence.
http://www.youtube.com/watch?v=bBU6loARLsc
It seems by 1988 Edberg had pretty much given up the flat serve and were experimenting with extreme, backhand-like grip. The logical explanation would be that he felt this combination gave him better chance at winning more points.
http://www.youtube.com/watch?v=A3QogooJIzc
Edberg in 1989 had a very abbreviated take back which I felt was awkward. It was a very loopy shot and seemed to lack sting. Becker seemed to like it at Wimbledon. Edberg came close to bagging two GSs but no cigar. It must've been disappointing.
http://www.youtube.com/watch?v=CgUljngu2do
Edberg towards the end 1989 had given up the abbreviated take back but his grip was getting more extreme. This form pretty much stayed until his second US Open win inm 1992.
http://www.youtube.com/watch?v=yB9Xluy2mHc
http://www.youtube.com/watch?v=-mRVUOixvG4
http://www.youtube.com/watch?v=9vZpMF4uGs0
Edberg's style would again undergo some major change in 1993. He was keeping his hitting arm pretty flat during take back as if to create more leverage. I for one think his S&V game reached its peak in 1993 depite his less than stellar results.
http://www.youtube.com/watch?v=NtEKxVV_-s0
Edberg seemed to be trying to serve harder and harder towards the end of hi career. Although his serve was still solid and was able to use the second serve to attack the net, his aura of serving greatness seemed to have dissipated.
When Becker served first serves close to 60%, he was virtually unbeatable. When Edberg served first serves around 70%, he was virtually unbeatable. As Edberg's grip turned more extreme, it was obvious he wasn't going for speed. Tennis saw extremely diverse styles among players during Edberg's era. Had Edberg been born just 5 years later, I'm sure we would've seen a very different Edberg than the one we got to know.