Certainly the two media coexist extremely well, because they are complementary in many aspects of compositional process, realized result, socio-economics of availability, presentation venue, etc.
If you have an orchestra, why would you need electronic sound sources? To do whatever an orchestra can’t, because by now they are cheaply available to all, or because you want to work on and interact with actual sound the way a painter or writer does on the final work rather than writing a complicated set of instructions in a limited imperfect descriptive language, or to explore sounds unheard.
If you have electronic sounds, why would you need an orchestra? For love of these sounds, to be part of a great tradition, to do music as part of a social framework instead of alone, to attain the professional "legitimization," or because of the subtlety and refinement that only centuries of evolution of an artistic medium can produce.
The few times I’ve tried to combine the two media, I have not personally found any aesthetic benefits not more easily attained within either medium alone, but that is strictly a subjective experience. For other composers there may well be very good reason to put these media together.
Just as some people work best in words without music, others in music without words, and others are at their best combining text and music, in the case of the combination of acoustic and electronic sounds, as in most other aspects of art, the best work is that which is entirely true to the character, needs, and visions of the unique individual composing it. In every way that the variety of music means can be increased, there exists more opportunity for the individualistic and personal to be accommodated and for a wider range of inner experiences to be explored, authentically expressed and communicated.