A statement of intent to fans and potential commercial partners. Symbolism has value, as does positive press, as does having as attractive an image of the club as possible to sell.
Second, a better stadium means a better experience for the people who attend, be they match-goers or tourists. Because, unsurprisingly, people tend to like being in nicer places and spending money on nicer things. And typically fans should be in favour of people having a better experience when they visit United's stadium, at least if they're not "real fans don't need that stuff" snobs.
Third, and probably most importantly, there's a lot of money it in over the long term. I mentioned on the previous page some of the developments planned for stadiums going forward, including the integration of 5G tech, augmented reality and virtual reality. All of which offers avenues to make a lot of money and not just from the fans in the stadium itself. Or at least it does if you have the capability to take advantage of it. And where a stadium like Spurs' already has that 5G capability, ours currently struggles with basic wifi.
And the club making money is good for the club and good for the team. Because money drives what happens on the pitch.