conversion to db stretches your backscatter intensity over a new value range by applying logarithm. That means, large values are shifted towards the mean and the low backscatter range (>0.2) is stretched. Consequently, your image has different contrats and some things are easier to identify.
You have to note that full backscatter (100% received signal) is at 1 in the normal image (power scale) while it is 0 in the db image (log scale).
Please also see here: Sigma0_VH vs. Sigma0_VHdb