You're showing pictures of a part, and pictures of a drawing view. I'm not sure you're comparing apples to apples. Creating parts with lofts and sketches in drawings are two completely different things.
Am I missing something?
If you want to have more control over the shape, then you would probably make sketches on intermediate planes that match the intermediate shapes in your sketch. Those sketches become profiles in the Loft.
When you added splines between the two planes in the tutorial, you were setting up for the following step. This (following) step is adding relations to the splines to define them withing the sketch and create the guide curves for the loft.
Splines are one of the tools that really lets us 'color outside the lines'. They start out completely pliable in all dimensions. So, when you add a tangent relations from the circular construction line (and the verticals) to the spline, it then conforms within those geometric constraints; creating a radius that smoothly transitions from the flat top to the edge of the lower sketch.
To answer your final question more clearly, the 'guide curves' are providing a boundary or frame from which to skin the part. The software essentially routes the surface via these lines, taking the most logical path between.
Hope this helps...