One could debate much on the origin of the pleat and whether it serves for function or pure visual aesthetic.
A distinctive crease or fold of fabric, gathered at the waistband and tapered down the hip- the pleat does in fact have a purpose.
The first is to allow for expansion- whether that be accommodating a little extra weight of the owner- or even allowing items to placed into pockets without creating excess bulge. Think of them like an accordion that retracts and expands.
The second purpose of pleats has more decorative appeal, in creating a bit of visual interest in narrowing the hip area, whilst tapering into the line/crease at the front of the trousers down to the bottoms- adding to the dimensional aspects of the garment.
To draw comparisons between tailoring and architecture might not be a far-fetched conversation. We are after-all turning a one dimensional fabric into a 3 dimensional piece to be worn. Much like structures serve a utilitarian purpose in providing shelter- yet the beauty of architecture accounts for space, light, shape, and planes that marry beauty and function.
No matter your preference- pleats will always have a place in our hearts at X of Pentacles and perhaps in providing you a tidbit of info, will find their way into yours as well.