From the user's point of view: they just want to run the thing, and as quickly as possible. If multiple programs want to use the GPU, then the OS and/or the driver should figure it out.
The reason for this flag to exist in the first place is that many of the models are larger than the available VRAM on most consumer GPUs, so you have to "balance" it between running some layers on the GPU and some on the CPU.
What would make sense is a default auto option that uses as much VRAM as possible, assuming the model is the only thing running on the GPU, except for the amount of VRAM already in use at the time it is started.
What I don't understand is why it can't just check your VRAM and allocate by default. The allocation is not that dynamic AFAIK - when I run models it all happens basically upfront when the model loads. ollama even prints out how much VRAM it's allocating for model + context for each layer. But I still have to tune the layers manually, and any time I change my context size I have to retune.
Your window manager will already have reserved its vRAM upfront so it isn't a big deal to use ~all of the rest.
It should have a heuristic that looks at available VRAM by default, but it does not. Probably because this is vendor specific and harder than you would think, and they would rather not use external libraries.