Looks like they're raising money because they can (good valuation), but little seems to be said about why (especially given that they still have money from the last round)
Perhaps Twitter is forecasting a future environment where raising cash will be more difficult or under less favorable terms.
That happened at my last company. Management sold a lot of bonds with stock warrants although we didn't need it at the time. The market tanked and we would have been unable to raise additional cash. That bond money kept the company going a little longer.
