Thermal underwear can be a purely insulating under layer, but they don't usually wick moisture away from your body as well as base layers.
Base layers are designed to wick moisture away from your body when being active. This helps keep you comfortable, even when wet. Some base layers like the woolpower 200 range are very warm, whereas thinner base layers like the Rab Pulse, the Paramo Cambia short sleeved, and the Woolpower light range will keep you comfortable in hotter weather.
A base layer is essential in an outdoor clothing wardrobe, as without the wicking properties your midlayer or shell layer will not be comfortable, even if they are good quality items. Some make the mistake of wearing a cotton shirt with waterproof jacket, which will leave you uncomfortable as the cotton shirt will hold sweat close to your body and make you cold.