Both FolderMount and Link2SD require root.
If you want to use all the features of Link2SD (i.e. linking apps and internal data to SD card), then yes you do need a 2nd partition on your SD card. But if you don't have that 2nd partition, you can still use all the other features, notably the ability to "move" some apps to your SD card using the native Android method, and the ability to link external data and obb files to the SD card.
If you were able to root your phone, then partitioning your SD card should be a piece of cake. Here is a guide:
http://www.noypigeeks.com/android/tutorial-a-guide-to-link2sd/
And if you don't have a PC handy, you can use this app to partition the card while it's in your phone.
https://play.google.com/store/apps/details?id=com.sylkat.AParted&hl=en
To learn more about Link2SD, and the benefits of having the 2nd partition, go here and do some reading:
http://www.link2sd.info
Since Link2SD added the ability to link external data/obb files, I don't think there is anything that FolderMount can do that Link2SD can not. However, I can still think of 2 reasons why someone might want to continue using FolderMount in addition to or instead of Link2SD:
1) If you are only interested in linking external data/obb folders, it could be argued that FolderMount provides a simpler UI for that purpose.
2) The free version of Link2SD doesn't offer the ability to link external data/obb folders. The free version of FolderMount will allow you to mount up to 3 data/obb folders.