It can indeed (usually) be turned on and off at will, which is more-or-less equivalent to a traditional camera’s shutter speed.
TL;DR: My mirrorless Canon R10 body has both a physical mechanical shutter and an electronic shutter, and you can choose between the two at will.
There are some additional considerations, one of which is tradition in that early gen digital cameras worked in much the same way, mechanically, as their film counterparts did simply due to ideological inertia. And for digital SLRs (true SLRs, not modern mirrorless cameras) a shutter is necessary due to the physical layout of how the camera itself takes pictures. When the prism is flipped from the viewfinder to the exposure position, the shutter needs to be closed at least for that brief moment so you don’t get a smear of junk reflection imagery swept across the sensor, which would then become part of the picture.
Also, not all cameras, especially early and/or cheap ones, can capture a whole image from their sensor all in one go, so attempting to implement a purely electronic “shutter” may introduce motion artifacts on pictures of moving subjects.
All digital cameras use an electronic shutter scheme when recording video, also.
If you’re asking why they need variable speeds, it’s for the same reason as a film camera. Only so much light can be gathered by the sensor over a certain amount of time, and longer exposures are required to gather enough photons to form an image in low light conditions, and vise-versa. The sensor doesn’t “know” what it’s capturing, it simply dutifully reports – with some degree of inherent error and variability – what amount of light hits each of its cells.
It can indeed (usually) be turned on and off at will, which is more-or-less equivalent to a traditional camera’s shutter speed.
TL;DR: My mirrorless Canon R10 body has both a physical mechanical shutter and an electronic shutter, and you can choose between the two at will.
There are some additional considerations, one of which is tradition in that early gen digital cameras worked in much the same way, mechanically, as their film counterparts did simply due to ideological inertia. And for digital SLRs (true SLRs, not modern mirrorless cameras) a shutter is necessary due to the physical layout of how the camera itself takes pictures. When the prism is flipped from the viewfinder to the exposure position, the shutter needs to be closed at least for that brief moment so you don’t get a smear of junk reflection imagery swept across the sensor, which would then become part of the picture.
Also, not all cameras, especially early and/or cheap ones, can capture a whole image from their sensor all in one go, so attempting to implement a purely electronic “shutter” may introduce motion artifacts on pictures of moving subjects.
All digital cameras use an electronic shutter scheme when recording video, also.
If you’re asking why they need variable speeds, it’s for the same reason as a film camera. Only so much light can be gathered by the sensor over a certain amount of time, and longer exposures are required to gather enough photons to form an image in low light conditions, and vise-versa. The sensor doesn’t “know” what it’s capturing, it simply dutifully reports – with some degree of inherent error and variability – what amount of light hits each of its cells.