If you want to play missions on higher difficulties, you need to rank up in Attack on Titan Revolution. It is very important to level up because it allows you to fight stronger titans. If you are wondering what the benefits are, then the biggest reason to do it is to get better rewards. If you play on Aberrant difficulty, you have a chance to get Attack Serum which allows you to perform Titan Shifting. This was just an example of why you should level up, and here is what you need to do to get it.

You need to upgrade your gear in AOTR to increase your rank. When you start the game your rank is E-, and the missions you get here are easy. But the rewards you get are also limited. So here are the gears you need to upgrade and how to do it:

Another benefit of a higher rank is that your gear performs better. As you upgrade your gear, you will see an increase in damage, speed and other aspects, making the playing experience even better!

Since you need it a lot for upgrades, the best way to get them is to create a lobby for missions that have a Gold Boost. This will give you more gold every time you complete a mission, which should speed up the leveling process.