Why? It helps during soldering.
Why? You may want the connectors to be touching the enclosure. Some connectors have springs, others have panel around them.
Why? So you do not forget about important stuff, so you do not have to look for the information again and again
Why? If there are no mounting holes, how you are going to fit it into enclosure or a system?
Why? Because if you write it manually you can make a mistake.
Why? It makes manufacturing easier.
Why? One VIA only can carry limited current (e.g. 1 - 1.5A).
Why? Just moving your table up may stop your back pain.
Why? If you use a generic resistor schematic symbol, then always when you generate a BOM (Bill of Material), you will have to manually edit it.
Why? You may be surprised how the first revision of a simple circuit may not work the way as you are expecting.
Why? You can very simply see on what page(s) the signal continues.
Why? It’s a very simple way to identify power nets (especially in layout), makes it simple to filter or order power nets , it’s very simple to use them in rules
Why? It improves your design and layout.
Why? Crosstalk depends on speed of rising / falling signal edge (how fast signal changes from low to high or high to low).
This is Lesson 7 of Learn to Design Your Own Boards Online Course.