Asset Management Within an Investment Bank2

Hello,
I am currently getting acquainted to the activities of an Investment Bank, particularly within the S&T Division. I am having trouble understanding the exact role of the Asset Management division within an Investment Bank.
My understanding is that the Markets division (i.e. S&T etc.) conducts trade execution, market making and occasionally prop trading. These activities are conducted for the buy-side (bar prop trading), i.e. asset managers, pension funds, corporates.

2 Questions arise from the description above:
1. Do the Asset Management divisions of Banks solely use the services of the sell-side of the same bank, or is this prohibited? In other words, is the Asset Management division of a Bank also a client of its Markets division? Are the clients of the Asset Management division of say GS the same as those of a major Asset Manager like BlackRock?

Can Asset Managers, Hedge Funds etc. only trade through investment banks? If not, why do they do it, given the commission of brokerage fees? Is there cutting-edge technology that banks provide that cannot be rendered by, say an Interactive Brokers, or does Interactive Brokers even execute trades through an Investment Bank? In other words, what leverage do investment banks to intermediate trades?
Any help is greatly appreciated.

SAbru