What can a speedlight be used for besides portraits?

Do I need a speedlight if I do not do people photography or portraits?

Depends. But yes, speedlights can be used for more than portraiture.

How else could one use speedlights?

Other uses for flash include:

• High-speed photography. Flash bursts can be much faster than top shutter speeds on camera bodies; e.g., bullet strikes
• Macro photography. The thin DoF caused by macro working distances can require smaller apertures and the magnification may require faster shutter speeds to alleviate camera shake blur. Flash can help with both.
• Product/still life photography. Just as editorial shooting can benefit from strict control of lighting ratios, so can product shooting. Ditto subclasses of product photography such as food photography.
• Stroboscopic effect. Similar to compositing several images to show a series of poses in a motion arc, you can do the same thing with a flash that's set to MULTI/stroboscopic mode so that it fires at regular intervals during a long exposure.
• Motion trails. 2nd-curtain sync flash can cause a motion trail behind a subject in motion.
• Light painting. During a long exposure, multiple bursts from a flash can be used to cover very large objects/areas (e.g., cars, trees, cliffs) so they can be light-painted.

Light is light, and whether it comes from a strobe or a continuous source, you can pretty much use it the same way. The advantage of a flash/strobe is that you can get a lot more light from one than from most continuous sources.