Lenses are worth the money you spend on them. At the risk of repeating myself, let me show you why.
A good lens focuses fast. It is well-built and strong. It has little aberration and edge distortion. It is silent. It has better coatings and resistance to flare. Most importantly, it has a larger aperture (a smaller “f-number”), hence more glass.
But also very importantly, it is sharper.
Look here. Click on this image and then click on “orginal size”, and then view it at original size. Make sure you follow all those steps.
(24-70 lens at 24mm, at f/8, 1/125th second, 100 ISO; using studio strobes).
When you do that, you basically get to DNA-level.
“But I do not want my face to be so sharp!”.
Yes you do. You want your eye, eyelashes and soon, to be sharp. Skin you can blur later if you wish, but the basic image must be sharp.
And that is why a good lens (e.g. in the Canon-world, an “L”-lens, where “L” stands for “Luxury) is worth every penny. (And they cost a lot of pennies – but the lens will last you twenty years, both technically and in economic terms).
I am repeating myself, I know – but this is important. In lenses, there are few shortcuts (except an affordable 50mm lens, which is why if you do not yet have one, go get one now!)