The answer to that really depends upon how often you’ll be building or modifying anchors on the sport routes you’ll be climbing.
Even so, you don’t necessarily “need” slings to build an equalized anchor. You can always sacrifice two quickdraws to do that, clipping one to each bolt and having your rope go through the two carabiners that are now hanging down.
The problem with this, is that it doesn’t leave you with very much extension.
If the bolts are too far apart, then the only way to build an anchor with quickdraws is to use four of them. If you use three, you’ll not distribute the load evenly across each bolt. The shorter side of your anchor will take the brunt of the force if you fall.
However, think about this: you can achieve the same thing with a single sling and a locking carabiner.
Now you see why climbers carry slings.
A sling and a locking carabiner is always preferable, even in the case where you’d only be using two quickdraws to build a basic anchor. A sling and a locking carabiner weigh less, cost less, and allow you to equalize three or more bolts without having to sacrifice any more of your gear.
So, this is where it gets back to the question of whether you really “need” to carry slings on sport climbs. While the answer is technically “no,” if you value your safety, your gear weight and your money then it’s absolutely worth carrying at least one sling with you on all your climbs.
After all, they weigh practically nothing, and don’t take up much space at all on your gear rack – especially dyneema ones, because of how thin they are.
Now, before you rush off and buy a load of slings, take a look at this post here where I explain which types of slings are the best ones to get.