Influence of side quests on the main storyline. (Star Wars: The Old Republic)

There is a so-called "research mission" (or something like that, in the original exploration mission's), it is clearly said about them that they are purely for a deeper study of the game, do not affect the plot at all, well, and there is no profit from them, except pure farming from them. But here's the question, do additional quests (not class and planetary, which are indicated in purple) affect anything?
PS The off site says that they are important, but for what exactly? I have come across many quests that do not differ from research missions at all except for a reward, and seem to give nothing at all. In general, either I do not understand something, or they will be needed for the addition of The Fallen Empire, since I have no other explanation.