1. When you use the projector screen on a daily basis. Apart from the convenience, the motorized projector screen will hold up better after continuous use of the rolling mechanism.
2. When the bottom of your projection screen is just above your equipment rack when rolled down. It is always best to have a bit of clearance to pull the screen down when rolling it down. An electric screen can be set to stop exactly where you want it, every single time. By having the complete control over the stopping point, your can be assured that your screen will not interference with the gear on you equipment rack.

Don’t think I am against manual screens , here are some instances in which they may be better for your application:

1. When cost is a major factor in your selection. Manual projector screens will generally be significantly less than their electric counterparts, if all other factors are equal.
2. When weight is a factor. Electric screens are heavier and may require mounting to more secure surfaces.  A manual screen can often be screed right into drywall with some anchors.