Strength allows you to move rocks across the field (which was replaced by Machoke's enormous, powerful arms in Sun and Moon) and is a decent move in combat. Strength is a standard move with 80 basic power but no secondary effect. It can be combined with other moves to increase their effectiveness. In terms of combat, strength gives your attacks more bang for your buck because you can hit harder things with them. This can be useful when trying to defeat strong opponents or when trying to do a lot of damage quickly. However, it doesn't do anything else relevant such as increasing your attack speed or giving you access to special attacks. Also note that since rocks don't have any defense, hitting them with enough strength to cause damage will usually kill them. As far as I know, there are no instances where having more strength than necessary would be beneficial. If you need to lift something heavy, use one of the other moves instead.