At low rpm's engine can only produce so much power.
Torque converters allow engine to be spinning at higher rpm's thus producing more power, while the drivetrain is spinning at low rpm's.

Thus what you think of fitting a torque converter to a manual transmission car?

Of course the torque converter must include a clutch, both to allow smooth shifting and to provide lockup as lockup-torque converters do to prevent slippage.