Q:

are SLI bridges necessary for general purpose GPU computing?

I looking at computer configuration with an option for adding a SLI bridge. The intended use of the GPUs are for general purpose/scientific computing, and not games.
My question is, will having a SLI bridge be helpful in improving performance?
According to wikipedia, SLI bridge seems to help solve bandwidth issues associated with rendering frames. I am just wondering if that also applies to generous purpose computing. 
-- EDIT --
I haven't used SLI before. So one additional question is, is a separate SLI bridge necessary at all to use SLI or does it come with motherboards that support SLI?
I have this question because when configuring computers online, I saw options for SLI bridges, e.g. here, where you can choose 
 "Dual Card (SLI)" (GTX 970) in the "Video Card" category, and
 "none" in the "SLI bridge" category.

I think this is the default I saw, which makes me wonder if choosing "none" vs the other options such as "EVGA Pro SLI Bridge V2" 2-way or 3-way matters at all, in terms of general purpose computing.

A:

If you want the two GPUs to share data, then SLI Bridge is recommended.
IIRC, it is actually possible to share data without SLI Bridge. But this is slower, as data must flow from 1 GPU to the PCIe then to the CPU then back to the other PCIe then finally to the other GPU.
If your purpose does not require sharing of data between the GPUs, then you can make use of the power of the GPU without using the SLI Bridge.
So, to answer your first question directly:
SLI Bridge will not improve performance if data is not being shared between the GPUs.
For your second question, whether or not the SLI Bridge comes with the Motherboard depends on the Manufacture (whether or not they are generous in giving you one). Sometimes, GPU manufacturers also provide SLI Bridge with their GPUs. Usually, if you need 3-way SLI Bridge, that is when you need to purchase one, as most manufactures do not include those with their GPUs (unless, of course, you buy them as a bundle or specially designated set).

