A good modern SSD should have a decent garbage collection algorithm built into the firmware, but not all of them have one. If you are going to use RAID, get drives that have hardware GC.
The issue is the SSD has no idea is a file has been deleted. That is because when you delete a file the OS doesn't even notify the drive. The OS simply records the delete in the file system. With magnetic disks this is a non-issue because there is no overhead/cost in writing over existing data. As a side note this is how utilities can "undelete" files. It is also why if you have a drive failure you should boot from another disk and only read from the corrupt/damaged drive.
Since the SSD doesn't know the file is deleted it can't do the "expensive" erase operation until the OS writes new data to the block. The presence of good GC doesn't change the fact that without TRIM the SSD simply doesn't know the OS "thinks" the page is empty.
The TRIM command is designed to enable the operating system to notify the SSD of which pages of data are now invalid due to erases by the user or operating system itself. During a delete operation the OS will not only mark the sectors as free for new data, but it will also send a TRIM command to the SSD with the associated LBAs to be marked as no longer valid. After that point the SSD knows not to relocate the data from those LBAs during garbage collection. This will result in fewer writes to the flash, reducing write amplification and increasing drive life.