Note that there are plenty of modelling techniques that will let you change rest poses. The reason to avoid doing all that is because you might find it difficult at that point to understand your character and their proportions. All of that places your rest pose more squarely in the middle of likely deformations. You can rest model the legs spread, duck feet, knees more bent, clavicles aimed slightly toward the sky. You can work around that, but it's not as convenient.

In Blender, the biggest is how IK behaves, where it is far simpler to pose a model with realistically bent joints if you want to use IK on those joints. There are other elements to choosing poses. (90 degrees, plus or minus, is also a place where you can see some problems with some rigging techniques, so if you can avoid that by pose choice, you do it.) You get terrifically ugly interpolation through the 180 degree mark, so you want that mark to be the least likely deformation you're going to see, and you choose your rest pose on that basis. In the extreme case of this, we could be imagining organic models where we wanted to allow plus or minus 180 degrees rotation, using volume preservation (quaternion skinning).

So your edges don't deform as much in A, your UV doesn't stretch as much, etc. Closing your arms is a 90 degree rotation from a T-pose, but only like 45 degrees from A pose.

The main reason to prefer an A-pose is that any reasonable deformation to the model is closer to the rest model.

And even if you're box modelling, working at an angle to world axes becomes less trouble the more tools you learn and use.) If you sculpt and retopo, you're not modelling by extruding loops out in various axes. (But for a lot of styles of modelling, it doesn't matter whatsoever. For some styles of modelling, that can be easier. The main reason to prefer a T-pose is exactly what you've experienced: the arms are more or less aligned with world axes.

Daz t pose