Why Are Table Scans Bad?

At my work, I am often told that I shouldn't do a table scan because it could negatively impact the database and the app using the database. What exactly do they mean by "negatively impact"? One negative impact I can think of is that a table scan would use up a lot of the disk's time, blocking other requests from using the disk. Is there a way to do a table scan without any negative impacts? Sometimes I need to do a table scan for one off checks (for routine things, I would of course make an index).