There are two reasons to consider getting a separate 50mm lens.
Yes, you already have a 50mm lens when you adjust your zoom lens to its 50mm focal length.
But, a separate 50mm lens is better in low light, and if you want to blur backgrounds more.
A 50mm f/1.4 lens lets in eight times more light than does the typical zoom lens.
That's three stops more light.
With a separate 50mm lens you may not have to use flash for indoor photography.
Your night photograph exposures don't have to be as long, and are therefore have less noise.
You may encounter the phrase fast 50.
Fast denotes a lens that let's in lots of light, such as
Digital SLR cameras have far more depth-of-field than film cameras.
Point-and-shoot cameras, have far more depth-of-field.
You can't blur backgrounds as much as you may like.
Sharper backgrounds are especially onerous when doing a portrait.
Yes, your camera has a face icon that is supposed to blur the background.
But, physics is physics.
The sensors of most digital cameras are smaller than film.
Therefore, there's more depth-of-field.
With a 50mm lens opening of f/1.4, you have three stops less depth-of-field that your zoom at f/4.
That's great when you're doing a portrait, for example, and you want to blur the background.
When photographing a subject that's close, at f/1.8 or f/1.4, focus carefully.
The depth-of-field is shallow.
For example, if you're doing a headshot, the camera may focus on an ear and not on the eyes.
Nikon sells two versions of a 50mm f/1.4 lens.
The newer version has slightly better specifications, but it costs considerably more than the older version.
Nikon also sells a 50mm f/1.8.
A lens at f/1.4 lets in about two-thirds of a stop more light than does a f/1.8 lens.
So, the f/1.8 lens doesn't open as wide as does a f/1.4.
However, the cost is far less than f/1.4 lens.
Nikon 50mm f/1.8D Good value
Nikon 50mm f/1.4D Old version
Nikon 50mm f/1.4G New version, but $$$
Both Nikon and Canon sell 50mm lenses with different widest apertures: f/1.4 and f/1.8.
A 50mm lens with a widest aperture of f/1.4 lets in about two-thirds of a stop more light than does a f/1.8 lens.
However, for this added amount of light gathering ability, you pay much more.
Besides Nikon and Canon, consider the Sigma 50mm f/1.4 lens.
Here's a review.
Sigma 50mm F1.4 EX DG HSM Lens Andy Westlake
Check your camera manufacturer's website for compatibility information.
Here are two charts for Nikon lenses.