A couple reasons. Monthly releases is how I reassure my backers and fans that I'm regularly making content for the game and moving it towards completion. It also lets me advertise it more frequently (although I guess recently I've shifted away from that to focus on working on the game.) Finally, it lets me track how long I've been working on the game (release 66 now, so I've been working on LD+ for 66 months, or 5.5 years straight.)
Generally speaking, if you've done everything in the game so far, it's not a bad idea to skip a few releases and maybe only play every couple months or so. That way, you can sit down and enjoy a lot of the game at once, instead of having to wait each month for a route's continuation.