Why shouldn't I use a direct connection from 5v pin to an input digital pin, and separetelly only by a switch, instead of adding a resistor and connect to ground that circuit?
You could... but the usual convention is to do do it the other way around. That's what people expect... and it's a good idea not to change things randomly.
There's some other circuit designs where switching to ground works much better - very long wires, electrically noisy environments, etc. This doesn't really apply to Arduinos but again it's better to use one design everywhere, not pick one randomly. That way the people who have to look at other people's circuits don't have to spend five minutes figuring out how basic things like switches are wired up.
PS: You don't need a resistor when connecting to ground, there's a resistor inside the chip. You would
have to add an extra "pull down" resistor if you were going to do it your way and connect to +5V.