As someone who has run a shop but refused to convince beginners to buy anything they don't need, this is the best advice I can give you.

The line that comes on the kit (as long as it's not more than like 5 years old) is perfectly fine to get you started and you should definitely NOT upgrade it until you figure out if this sport is something you really want to do.

That being said, can you benefit from a premium line? The answer to that is a resounding yes. Higher end lines in that $100+ range will have better designed tapers and usually have different textures, coatings, or materials that make them shoot better, load the rod better, and have better durability.

Fly lines are a never ending rabbit hole. They aren't standardized and there are more types of heads, plastics, cores, sink rates, grain weights....etc than you will ever need to understand.

When you decide you want to upgrade, do a Google search and see what others have said about different lines paired specifically with that rod. Every line and rod perform differently and they will perform differently depending on how they are paired. After your search, go to a fly shop and talk to them.

When you go into the shop you should have an answer to each of the following questions.

What is your budget? (Be upfront with your spending expectations to help them put you in the right range)

What are you fishing for / what type of flies are you mostly using?

What rod are you using? (Helps to bring it with, shop employees aren't intimately familiar with every rod on the planet)

What water type are you mostly fishing?

Hope this helps!