I've never played on maps, but they seemed to of offered an easy baseline for building and general grounds for gameplay with little to no issues. People aren't too bothered to make several edits (i.e. just building in the map), though having to make almost everything from scratch is a burden and why some resort to using something like Collab City or Kobble Island, especially if they wish to test out their gameplay ideas. Imagine if Minecraft or GMod were always based in a flat world, and you were forced to either use small pre-built structures or make your own from scratch. A lot of fun screwing around in sandbox is thanks to foundations already provided, whether it's a capable physics engine or random world generation.
It wasn't the best move for gameplay, though that doesn't mean we're powerless to develop and provide options equally as functional; creating mods like Static Maps or releasing your builds helps a lot (again, imagine if there were little to no community maps provided for Source games). Aesthetically, it pushed the game out of what looked to be a remnant of 90s RP/G games, easily attracting a larger crowd. The default shadows don't look amazing, but the overall image given to Blockland has significantly improved. To some, showing capability in some sense is important.
My thoughts on Blockland as a general sandbox game. Even so, it focuses on building specifically, which it's still great at.