When I entered this line of work I found the hardest part of learning how to build links was figuring out which strategies were sound, and which were a waste of time.
Time could be wasted for a plethora of reasons.
1) The tactic you are using was completely valid two years ago, but now due to changes in algo and environment they are no longer worth your time
2) They are so despised by fellow webmasters that they may not be worth your time
And the list goes on.