I believe the 488MKII is capable of providing phantom power (+48V?) for two mics,eliminating the need for an outboard pre-amp. You may want to check your manual to be certain.
Take a look through the previous discussions on compressors, you'd find them informative of their merits. What it comes down to is what you want to record (guitar, vocals, etc.), and if you can record it without additions to your signal chain that is the generally preferred method. So, while it may be nice to have for situations, it isn't necessary.
Good luck.