The love scenes act as a contrast and show a different shade of Terry's character - that he is a caring person who is capable of actually loving someone. Compare this to Johnny Friendly who playfully punches, teases and plays with Terry at the start of the film but only to "buy" him (by offering him half a buck) - I think the love scene shows that Terry is capable of loving someone without "buying" them and without using them for his own selfish or evil need (as Friendly does with Terry) - and it shows his moral and emotional development as a character
Also, the room in which the kiss between Edie and Terry takes place is the same one that Joey was summoned from at the beginning of the film.Also note the way Friendly's minions keep shouting "Terry" like they were shouting for Joey at the beginning. This shows the risks and consequences (i.e. dying like Joey did) that Terry is willing to take to be with Edie. His sacrifices and his yearning to be understood by the one person he cares for are feelings that most people would experience and so the audience is almost made to empathise with Terry and see him as a changed man. The love scene not only establishes Terry's and Edie's love but also confirms his willingness to be accepted as a better and moral person both by Edie and by the viewers.
I think the love scene also signified Terry's not being controlled by Friendly (or anyone) anymore and that new-found independence would also exalt him in front of the viewers. Though he was quite aggressive with Edie, he is quite a physically acting person and words don't really come well to him as they do to Edie (note the scene in the middle of that ship hold place where his words get muted by the sound of the ship horn) - so actions speak louder for Terry and in this case, it was one of the first scenes where he didn't let someone else control him (despite Edie shouting her ass off at him hahaha) and so that shows Terry's elevated character development compared to anyone else in the film
