Google probably acquired a bunch of domain names with .xxx extension to save themselves the headache of going after them at a later stage.
The most important of these is probably YouTube.xxx which is now owned by them.
The domain does not seem to redirect to YouTube.com right now which is weird.
In any case, Google probably has no use for the newly acquired domains. XXX domains just adds the headache for regular brands and companies to block another range of domains and it costs a lot more than other domain types.