[Captain Kwok]

Mainstrings.txt only controls how strings are interpreted. For example, you could change the name of the ability "Master Computer" to "Command Computer", which means you would have to change the Master Computer entry in Components.txt to use the ability "Command Computer", but it would still do the same thing as it did before. It's only really useful for making translations, such as if you were making a German version, you'd want to re-name all the abilities to German names etc. Alternatively you could re-name resource types but you wouldn't be able to add any new ones.