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Out of curiosity, what difference does it make in terms of handling/performance/comfort to have a more relaxed seat tube.

It's all about hip angle. If you raise the bars, you can then move the seat back without closing the hip angle. It affects weight distribution, putting more weight on the saddle and less on the hands, which might be more comfortable. Essentially, without changing the triangle formed by the three contact points, you can rotate the whole body by a few degrees either way. Tilted forward usually reduces frontal area, and thus aerodynamic drag, while increasing the weight borne by the hands and arms which can be tiring. Tilting back reverses all of the above. In the case of TT and Tri bikes, the seat tube is sometimes steepened without further lowering the front end, which is already aerodynamically optimised, and this has the effect of opening up the hip angle which affects the muscle recruitment in pedalling, which may either increase short term power output (TT) or reduce fatigue in muscle groups which are needed later fro running (Tri).

In addition to all of the above, the seat tube angle affects the set back of the saddle from the BB, which is a fit issue based on femur and foot length relative to total leg length.