first of all, there should be a good balance and investors should not be the only land owner in the US. I don't think many citizens are interested in renting their whole lives. This is not a list of Cons, and there are plenty of Cons that go along with investors.

That being said here goes:

1) investors convert distressed properties from low use to high use. Most people won't touch or don't know how to flip a property, Those that might want to try will frequently do it poorly or make major mistakes on code or design elements.
2) Converting bad properties into desirable ones is great for neighbors, communities and house value for everyone.
3) Investors create construction jobs, service jobs for cleaning/maintenance/lawncare that are frequently done by a homeowner.
4) Real estate transactions that create revenue for an investor turn into additional taxable income for the locality.
5) For non-instutional investors, it is a great inflation hedge, its diversification, and arguably a better source of reliable retirement funds than any other vehicle.
6) All real estate is technically an investment so where do you draw the line? From Builders, to lenders, to General Contractors, to brokers. Everyone is trying to make money at this. Each of them brings unique value to the market and owns a piece of real estate investing.
7) investors clear foreclosures, tax liens, and other stagnant real estate from the market and puts those assets and funds to better use. This adds inventory to the market.
8) investors help real estate velocity by adding additional buyers and sellers. The faster the homes sell, the quicker you get to sell your house when the time comes.
9) investors provide rentals to the market, a needed commodity. Without investors, you have no rentals. Rental properties provide vacation stays and eliminate expensive closing costs for transitory tenants.
10) Investors bring tourism dollars to local markets and all the ensuing economic benefits that tourism brings.