Why Learn Constellations?
To get to where you want to go!
The Sky is static, unchanging, fixed. It's also overflowing with stuff.
The maps of the sky are outlined by Constellations.
You can pinpoint locations if you can recognize the boundaries of smaller regions.
Like finding cities in a country, you can find them better if you recognize smaller regions in it.
For example, in the US, it's much easier to find cities if you know what state they're in and the outline of the states are in the map.
Finding a celestial target in binoculars or a scope is much easier if you know what constellation it's in and its location within that constellation.
For example, if you have binoculars or telescope and want to see the most glorious nebula in the Northern Sky, all you need to know is where the Orion Nebula is:
Constellations were "invented" using the brightest stars. They're the most universally (!) visible stars, so they're the best guide posts.
Asterisms are "star groups" that aren't "official" constellations. The Big Dipper is a familiar asterism, but only a part of Ursa Major. They make great guide posts as well. The Summer Triangle and the Winter Triangle are easy to find and get you instantly oriented to the sky.
Constellations and Asterisms help you find the interesting stuff in the sky - most of which are not directly visible with the naked eye.
If you can find a constellation you can probably point a telescope to the cool objects in it. The process is called Starhopping - going from a map to the sky.
If my earth map directions tell me I need to get onto Interstate 8, I need to look around in reality and find Interstate 8. I can then proceed with the directions.
If my sky map tells me I need to go to the middle "star" of Orion's sword, I need to know where Orion is in the sky before I can continue.
To contine the roadway/map analogy, Constellations and Asterisms are the highways to get you quickly close to your final destination. Star Charts are the local map to get you the rest of the way.
But how do I Starhop? I'm glad you asked: How To Starhop