Lobbying, especially at the state level boils down to a lobbyist being able to state the merits of his/her clients position to legislators who probably know very little about the issue.
In Indiana, we have citizen legislators. That means they have real jobs they go back to when the legislature isn't in session. Sure, some are retired and some even use their legislator's salaries as their primary/sole source of income, but by and large, legislators have other jobs.
Well, just like any person off the street, they have their own knowledge-base and they have their own pet issues, but there are hundreds of other issues, and thousands of pieces of legislation that come across their desks during session.
So a lobbyist's job is to voice the merits of a particular position on a subject about which, the legislator likely knows little.
If some random stranger comes up to you on the street and asks you to join his church/social group/whatever, you'll probably politely listen and then walk away. Same thing with lobbying. Lobbyists acquire credibility over time. Each time a lobbyist talks to a legislator, that's another occasion where he or she makes an impression. If a lobbyist misleads a legislator, when that legislator finds out, they are going to distrust future statements from the lobbyist.
Therefore, when a lobbyist speaks for his or her clients, they must, of course advocate their client's issue, but they must also ensure that they are straightforward and honest with a legislator when doing so.