A while back I was asking my twitter followers about achievements and their significance in a game.
The general sentiment was that achievements were an important feature that you should consider adding. Let me present you the results and explain why.
1. ...Make Players Stick to your Game
Gamers want challenges. So by providing them achievements they can complete you are also giving them a sense of accomplishment once they finished them. They will want to complete as many achievements as possible and play your game longer, too. Even better if you're also giving them rewards while doing so!
2. ...Make Players explore Your Game
When a player sees a certain achievement, they will spend time trying to do things that may help them get it. This gives them a very good reason to explore your game and scoop out all possibilities.
3. ...Add Replayability & Variety
In case your game has mechanics that allow usage of different strategies or play styles, achievements are the best way to let many gamers experience as many aspects of the game as possible. If you want to promote creativity, reward players for doing things differently. Having your hands on the knob called variety is, in my opinion, a very crucial tool of game design. I think Team Fortress 2 does this very well - even though in practice it doesn't always turn out so well!
4. ...Provide Guidance
Achievements can give important information to the player, such as how the game is being played or what else can be done in your game. Players will know what gameplay, items etc. to look after just by checking the list of achievements - if you are providing them with one.
5. ...Encourage Competition
Especially in online gaming communities, achievements are often worn as a badge of honor and proudly shown off to others. Paired with a leaderboard, that also increases the chance someone plays your game longer.
I remember playing too much agar.io trying to get on the leaderboard in the first place...
Achievements are a great way to enhance your game, even if it is a small one. They increase variety, add more challenges and makes players explore your game more. All of which that, if done right, eventually results in more fun and time spent playing.
For further reading, you can check this video on which types of achievements are the best ones (Youtube). Thanks for Mr. Aqua for showing me this, you know who you are!
Thanks for reading!