That's a great question!
To my understanding, "左侧" is usually used to point out stuffs alongside the road. For example, McDonald is located on the left side of the street（麦当劳在这条街的左侧，we don't say 麦当劳在这条街的左面）
.“左面” is used to point out the location of something compared with an object, including people. For example, the mountain is located on your left side（这座山在你的左面）。This building is located on the left side of that building. （这栋楼在那栋楼的左面）