There was already a preview program for the Master Chief Collection when 343 Industries was fixing the Xbox One version, but it's giving the concept another shot now that the PC ports are on the way. It's launching a Halo Insider Program that will give you a chance at playing pre-release versions of all Halo games and services, whether they're for console or PC, in return for feedback. You're not guaranteed to get into every test, but it may your best shot at playing a future Halo game in advance.
The 343 crew didn't say just when it would release the first preview, although Halo: Reach is a strong candidate for that preview when it's the lead title for the MCC PC introduction. Both Xbox and PC players will be asked to fill out occasional surveys. If you're an Xbox fan, you can also use the Xbox Insider Program's "report issue" tool to provide input.
There's no secret motivation behind the new program -- 343 knows the original MCC release had plenty of teething troubles, and it wants to avoid a repeat. This initiative increases the chances of a smooth launch, and might just rejuvenate overall interest in Halo.