The larger the number, the wider the spectrum view (3.8 MSPS would show 3.8 MHz of spectrum). But, the larger the number, the more processing power is needed. If you are on a low-processing powered device (i.e. Raspberry Pi, very old PC, etc) and are seeing performance problems, lowering this value can improve performance.
I've noticed that certain sample rates actually reduce the signal-to-noise ratio with my RTL. I stick to 2.048 MSPS, since it was the default setting and it works well. However I've noticed 3.2 MSPS (the highest rate for my dongle) works better for HF.