Critical Mass - most managed funds will have assets under management (AUM) in the hundreds of millions, this gives these funds the advantages of "critical mass"- the benefit of getting a higher return from the same instrument because of its size.
Let's take "time deposit" as an example.
You have 50,000 you want placed in a 30-day time deposit,
on your own, the interest you get will be proportionate to the size of your deposit which is 50,000.
A 500 million fund that only invests in "time deposits" would be able to negotiate for higher interest rate because of its massive size.
The key is to find a fund with investment objectives and parameters that matches your own, instead of investing DIY style, the investor will be better off investing in the fund to benefit from critical mass.