I just started exploring some Sentinel-3 Datasets, and I have found them to be much more complex than S-1 or S-2 (so many bands!). One thing that I cannot figure out is why SLSTR images are taken both in nadir and oblique orientations. From what I can tell from a cursory glance, the only differences are that the images are taken around 2 minutes apart from each other, and that the oblique image is narrower.
So if I understand correctly, the only purpose on Sentinel-3 is to have matching images with ATSR? There is no reason to use it if I am not comparing with ATSR?
Sentinel-3 made improvements over ATSR (the biggest improvement is just that S3 is newer and should last longer), see SLSTR:
The principal aim of the SLSTR instrument mission on-board SENTINEL-3 is to maintain continuity with the (A)ATSR series of instruments. The design supports this by incorporating the basic functionality of AATSR, with the addition of some new, more advanced, features.