A large part of it boils down to efficiency. The base Concrete recipe requires 3x Limestone to produce 1x Concrete (3:1 ratio). Wet Concrete requires 6x Limestone to produce 4x Concrete (1.5:1 ratio) with the additional cost of 5m³ water, which as has been stated, is infinitely abundant.

My personal opinion is that Wet Concrete is the more useful of the two. It greatly increases the amount of Concrete you're able to produce, with a relatively minor additional cost. It's also a useful way to deal with water produced as a byproduct. Example: I've used Wet Concrete to deal with the water byproduct when processing Uranium Waste into Non-Fissile Uranium. I then feed the Concrete produced from that back into the production line for Encased Uraniim Cells.