Anyone who makes campaign contributions expects some benefit to come of them. But there are many different forms this expectation can take.
Form 1: Donor goes to candidate and says “I’d like you to vote yes on Bill XYZ and similar bills. If you do so, I will give you money.” This is bribery, and is illegal.
Form 2: Donor finds candidates who already support XYZ and similar bills, for whatever reason. Donor gives money with no strings attached to those campaigns, but does not give money to campaigns of opposing candidates. Donor hopes that the money will enable those candidates to win, and so when XYZ comes up for a vote, it’ll get a majority. Meanwhile, opponents of XYZ are doing the same thing with the candidates who already oppose it for their own reasons. This is certainly legal, and if there’s any legitimate purpose served by campaign donations, this is it.
Form 3: Donor finds politicians who are still on the fence about XYZ. Donor gives enough money to the politician to get the politician’s attention. Representatives of the donor then go to lunch with the politician and lay out their arguments for why XYZ is a good idea. Meanwhile donors on the other side do the same thing, with the same politicians. The politician takes both sets of donations, and decides which way to vote. This is legal, and the part about laying out arguments is certainly legitimate and how the system is supposed to work, though it’s perhaps unfortunate that the way to get the politician’s attention in the first place is to give money.
Form 4: Donor goes to politician, and says “We’ve decided to give you money this year. We haven’t yet decided whether to give you money next year. On a completely unrelated note, here’s a list of legislation that we’d like”. This is legal, provided that the donor is careful enough to not make the connection explicit, but it’s still pretty shady, and lots of folks would like to find ways to make it illegal.
And, of course, there are also differences in the sorts of benefits donors expect to receive. Some donors are trying to get effects that they consider to be to their own personal benefit, while others will try to get effects that they consider to be to the benefit of the nation as a whole. And some, of course, will consider something beneficial to both themselves and the nation.