While some photographers like the natural view offered by an optical viewfinder, an electronic viewfinder brings the advantage of being able to see the effect of the exposure, white balance and Picture Style settings being applied. If you apply the Monochrome Picture Style, for example, the image you see in the EVF will be mono, while with an OVF it will remain colour. This means you can use the image in an EVF to assess whether your settings suit the scene and to be confident you will get the result you want before pressing the shutter button. That's especially helpful if, for example, the subject is backlit and you might need to use some exposure compensation.
In this way, an EVF is especially useful for relatively inexperienced photographers, because it enables you to see the effects of camera settings at the shooting stage, not just assess them afterwards. For many, it makes photography more intuitive.
Another advantage of an EVF is that it can compensate for low light levels, which means you always have a clear view of the subject. Conversely, with an optical viewfinder you're seeing the scene with the ambient light level, which means that in dark conditions it can be difficult to compose a shot or to focus.
On the other hand, because the image you see in an EVF has to be processed before it can be displayed, all EVFs suffer from some degree of lag. Although the latest mirrorless cameras such as the EOS R5
have EVFs with a refresh rate of 120fps and the lag is only a matter of milliseconds, this can still matter if you're shooting fast-moving action and split-second timing is critical. As technologies continue to develop, the lag is likely to get shorter and shorter, but an OVF works at the speed of light, which means in effect no lag at all. For this reason, many photographers shooting sports, wildlife or other subjects involving fast action still prefer a DSLR
In addition, when you're using an EVF you're actually looking at a small screen, and even though this has a very high refresh rate, an OVF can be more comfortable over a long period of usage. This means that if you're shooting wildlife or sports where you have to keep your eye to the viewfinder for a very long time waiting for the action to happen, an OVF could be preferable.