What are the differences between email or snail mail?

Email short for electronic mail, as we all know still uses an address and it doesn't physically cost you the money of a stamp to send a letter or pictures to someone, but it's not free. Snail mail could take a couple weeks to get across the country back in the day and now an email can go to 70 people in 40 countries instantly. In certain situations a handwritten letter is way more personal and still done by some people, but if I'm not mistaken I believe electronic Christmas cards, have even surpassed traditional ones. The information I found just says the eCards have hurt the greeting card industry they don't have exact numbers, because no real research has been done I'm assuming.
If we had invented computers and not come up with a system to communicate quicker than phoning everyone in a development teamm one person at a time, I couldn't imagine how hard it would have been to write a program or maintain a network. My first college degree in 1994 was in computer science. I learned programming when C++ was brand new and we converted the 386 PC lab to a 486 lab with ethernet cable replacing the coax cable for our Summer project. The largest group I worked on during a project was 5 and without email for communicating ideas after class hours I would've personally been divorced before I was married, because my girlfriend thought I was cheating on her just from the emails from a group member named Pepper.