As long as the valuation of the company is going up more than the dilution, the value of existing shares goes up.
In an early stage company that might mean raising a Series A where the valuation of the company goes up 100% and shares are diluted by 35%. Value still going up.
For an ultra late stage company like Uber that means ~5% dilution in this round and the total value of the company might go up 10-20%. The value of your shares is still going up.
For simple back of the envelope math, assume you invested 10k into Uber's seed round valued at 3.5 MM. Then we'll simulate some excessive amounts of dilution, so let's say our 0.28% stake is diluted by 30% 10 times (which is a ton of dilution and way more than would have reasonably happened).
You are now down to 0.007% of Uber. 0.007% of 65 billion is $5.1 MM. Not bad for a 10k investment.
Based on more reasonable dilution estimates my guess is a 10k investment in Uber's seed round is worth somewhere between 25-50 million.