Hello,
the 'Erase Script' function in mcOTA is provided in the rare instance that someone writes a script that 'bricks' a device. This can happen if you write a script that loops/bricks due to poor programming (yes, it happens!).
While working with mcThings devices/modules, you can overwrite existing programming by loading new scripts through mcStudio (either wireless via the gateway or USB serial connection with an mcDemo205). Loading new programming automatically overwrites the existing programming on the module/device.
There is no need to use mcOTA as a means to erase existing programming unless you have written a script that requires you to do so.
Does that make sense?