Tomatoes are typically peeled for sauce to create a smoother texture, removing the skin which can be noticeable and sometimes bitter. Salsa often benefits from the texture and slight bite the tomato skin provides.
The decision to peel tomatoes for sauce versus salsa largely comes down to the desired final texture and flavor profile.
Tomato Sauce: When making tomato sauce, the goal is usually a smooth, velvety consistency. Tomato skins, even when cooked down, can remain slightly tough and create a less refined texture. Furthermore, some people find the skins to have a slightly bitter taste that can detract from the overall flavor of the sauce. Peeling the tomatoes eliminates these issues, resulting in a smoother, sweeter, and more palatable sauce. The process of peeling often involves blanching the tomatoes briefly in boiling water, which loosens the skin and makes it easy to remove.
Salsa: Salsa, on the other hand, is often intended to have a chunkier, more rustic texture. The tomato skins contribute to this texture, providing a slight bite and visual appeal. The skins also help the salsa hold its shape and prevent it from becoming too watery. In many salsa recipes, the tomatoes are used raw or only lightly cooked, so the skins don't have as much time to break down. The slight bitterness of the skins can also add a layer of complexity to the salsa's flavor.
If you prefer a smoother salsa or are using tomatoes with particularly thick or tough skins, you can partially peel them by scoring the bottom with an 'X' and briefly blanching them before dicing. This allows you to remove some of the skin while still retaining some of the texture.