2. The moons aren't always visible (they're behind or crossing Jupiter).
3. The magnitude of the moons changes with the relative positions of Earth and Jupiter.
It's possible that Galileo was not the first to see them, but if you don't know what they are or how you managed to see them, you're probably not going to be able to get anyone else to believe you. The telescope allowed Galileo to track their movement and also to convince others, who could also look through a telescope and see them easily.
1. Jupiter is very bright.
2. The moons aren't always visible (they're behind or crossing Jupiter).
3. The magnitude of the moons changes with the relative positions of Earth and Jupiter.
It's possible that Galileo was not the first to see them, but if you don't know what they are or how you managed to see them, you're probably not going to be able to get anyone else to believe you. The telescope allowed Galileo to track their movement and also to convince others, who could also look through a telescope and see them easily.