The valve body is the brain of a transmission.
The Valve Body controls...
Which gear to apply at what time...
How fast or slow gear changes occur...
How much pressure to apply to each gear...
and can control when fluid is being circulated, and where fluid is being circulated inside of the transmission...
... ... .
What will changing the valve body do for you?
Fluid will circulate while in park, allowing trans temperatures to be cooler...
Gear changes will occur more sharply, not allowing as much slippage during gear changes.
"Line pressure" or, pressure being applied to the components, will be increased significantly, allowing your trans to hold more power without slipping.
The side effects are few, but can involve erratic part throttle shifts, and shifts that are too firm for your liking, although that can be tuned out very easily.
There is not any accelerated wear from using a aftermarket valve body. Running high line pressure, and opting for very firm shifts will not wear or break the transmission.
Merrick