There are several reasons to dim a T5, some may or may not apply to you.
To simulate the intensity of the sun throughout the entire day. Mine only peaks for about 2 hours.
The ability to increase or decrease overall intensity, without having to to raise or lower the fixture.
The ability to decrease the overall intensity for acclimation purposes.
The ability to fine-tune colors by dimming bulb groups.
If your fixture supports it, the ability to run a true moon cycle that varies in intensity throughout the month.