Provide full services to your mutual fund clients to forestall their investing directly or through a broker dealer. There's no need to outsource the finding of a custodian or trustee someplace else. Simply provide it in house.
Keep control of your captured assets. As a mutual fund company it makes better economic sense to keep all your assets under one roof. To refer your business to an outside trust company for the accounts that require this form of intermediation poses risk of loss of accounts.
New profit centers. By owning your own trust company you can gain profits from charging trustee fees and other fees to your clients, thus creating a new profit center for your mutual fund.