What kind of project can this tool be used for?
I think the author would explain best. Personally, I'm working on casting live earth shadows on a tiled map (link in HN profile) and lack an off-the-shelf tile set that contains all the data I need and in some cases the available tiles I use contain more data than I want. Also, the tiles cost $$$ for each tile load.
So long-term I am looking into generating and hosting my own map tiles.
Depending on your needs, tilemaker (https://github.com/systemed/tilemaker) may be the least painful way for generating tiles from OSM data with modifications to it. Or at least it seems to be the least painful way of doing that that I've found so far. The input data is manipulated by Lua code that you provide, and some reasonable default configuration for it exists. Your custom Lua code could pull additional information from other information sources should you wish to do so.