Qatar dont need United to buy their way into Western society, they already have solid connections with power in the West, own prestigious assets and have hosted the World Cup. They are in already.
But Shiek Jassim's father, the former PM and uncle to the current Emir with who he reportedly has frosty relations, who was reportedly too rich for comfort for the current Emir, who stepped down the day after the latter's ascend to power might, like Roman Abramovic before him, need the protection owning a premier brand like United affords him by catapulting his family to prominence.
If he is an insider on the fringe he might not feel secure should the former Emir die. Owning United affords him and his family the popularity or visibility that would (a) allow him to shift his wealth base from Qatar to Manchester and (b) make him and his family intouchable due to prominence they would get by owning United. He would give his family a platform and political to establish themselves in Manchester.
If his father's wealth was reportedly understated to an extent that the current rulers aren't comfortable with it they could very well afford United and sacrifice a bit of it to make United a springboard on which to establish a real estate and hospitality empire. And he may not be alone, there must have been a few corrupt former Ministers that are rich enough to form a consortium with him.
Since we are all speculating and all.