Good lands are the backbone of a well functioning deck. You can’t only rely on basic lands when running a deck made up of 2 or 3 colors. What could happen when your opening hand consists of 3 basic mountains but you need green mana to cast the other 4 cards in your hand? You could take a mulligan, but the results could be similar. Replacing a few basic lands with some dual lands or fetch lands could prevent scenarios like this from happening. There is also something to be said for the number of lands a deck should run as too many and you will be more likely to draw a land than a spell, or too few and you may not have enough mana to cast spells. Use the following for a basis when deciding how many lands to run in a deck:
40-card limited deck: 17 lands
60-card constructed deck: 18-25 lands
Fast deck (aggro): 18-21 lands
Slow deck (control): 22-25 lands
Combo decks can run anywhere from 12-28 lands depending on the format and their strategy
99-card singleton/commander: 34-38 lands
This is just for a basic guide. Some strategies or formats may require more or fewer lands, for instance I would never suggest running 25 lands in a modern deck, even if it was a control oriented deck. 20-22 is usually plenty for modern and one-half to one-third of those should be fetch lands most of the time.
This concludes the post on lands. If you have any questions, feel free to leave them in the comments below and I will do my best to get back to you with an answer.