First of all, it's not just opening up .NET. It's also many other cool things Microsoft has been doing: Linux on Azure, TypeScript, etc. etc.
Regarding .NET, it is likely it will benefit Microsoft significantly. First, because Microsoft would not do it without strong reasons. Second, we can guess at those reasons: Microsoft wants to grow its developer ecosystem. .NET has been limited due to being Windows-focused. Opening it up makes sense.
(Yes, Mono exists, but the .NET cross-platform experience still wasn't good enough. Not necessarily Mono's fault, but regardless, opening .NET can solve this (possibly at a cost to Mono).)
Regarding .NET, it is likely it will benefit Microsoft significantly. First, because Microsoft would not do it without strong reasons. Second, we can guess at those reasons: Microsoft wants to grow its developer ecosystem. .NET has been limited due to being Windows-focused. Opening it up makes sense.
(Yes, Mono exists, but the .NET cross-platform experience still wasn't good enough. Not necessarily Mono's fault, but regardless, opening .NET can solve this (possibly at a cost to Mono).)