Since you're new to all this, I'll give you the clear and simple reason for a traveler.
It's specific use is for upwind sailing only, and usually for racing.
When you're close hauled, or sailing as close to the wind as possible, you want the boom to be on centerline. That is, to have the back-end of the boom to be in the middle of the boat.
WITHOUT a traveler, you can come in on the mainsheet as much as you want, but the boom will always be off to the downwind or lower side of the boat.
WITH a traveler, you can tighten up the mainsheet to fill the sails nicely, and then move the traveler car up till the boom is centerline.
Then all you have to do is point the boat as high to the wind as you can without luffing the sails. THEN you'll know that that is as high as you can point the boat.
If it's just for daysailing and not racing though, and the only place to put the traveler is in the center of the cockpit, than I DON'T recommend you getting one.
It'll just get in the way. But if the traveler can be placed AFT at the very back of the boat, with the lines at the end of the boom, than a traveler is a good idea.
I highly recommend a Harken windward sheeting system.
You can move the lines simply and easily.
I won't get into why it's good here. My note is long enough!
Good sailing!
JW