You can combine it on 1 disk, if you wish. Not obligated to split.
The reason is to optimize the disk for the purpose. Journal is a capped collection that just writes in sequence. So less IOPS needed. And logs are logs, just adding information. And then Data, well, reading jumps around a lot, and writing sometimes also, filling up gaps that were freed. And while journal and log are written away on other disks, the data disk doesn't lose time on that. Every little bit can help on intensive systems. The next step then is Replication and Sharding to spread the load.
On the https://university.mongodb.com, you can get more information about this if you are interested. Following M202 MongoDB Advanced Deployment and Operations now, what offers some specific information to optimize.