Basically UA gives you 25% of your investment back as amenity funds (except $45K which gives you $15K in amenity funds). You get them pretty much immediately once your investment has been deposited and can use them for a few different things.

• Buying status for you/someone else that was posted previously
• Buying wifi subscription, E+ subscription, United Club subscription (usual prices, 1 amenity dollar = 1 USD)
• Round trip award tickets in specific fare classes (1 amenity dollar = 0.2-0.5 USD based on typical cash fares)
• Premier Access for a particular segment
• One time passes to United Clubs

As a single user (no companion bookings), the only uses I'd consider are the subscriptions that I don't already have, bumping from Plat to 1K, gifting Gold to someone who could benefit from E+, gifting a one-time pass, and in niche cases, bumping from Gold to Plat. The math is obviously a lot different if you're also booking for companions using your funds... wouldn't be surprised if someone paid $75K to get GS to fly Polaris with family year-round that they otherwise wouldn't get based on individual spend.