Nokia could essentially use past royalty obligations as a coupon in it's purchase of InterDigital. No need to pay InterDigital upwards of a billion dollars in expenses when with a few billion more could make a capital acquisition of the entire company.
Nokia could aid Microsoft on a different front against Samsung and Apple who InterDigital has, and will soon have, expired patent contracts with.
Nokia knowing the legal strategy of the collective (ie. where the dead bodies are) could turn informant against Huawei, ZTE, LG and Samsung and take their knees out to the benefit of Nokia's lover Microsoft.