Most, apart from the ones that look like they are only strong enough for show, are to keep the strut turrets in place when cornering hard, especially when using sticky tyres. If the strut turrets move, it alters your geometry, hence the tyres contact patch with the road, hence potentially less grip.
A good strut brace when needed will help make the car feel more responsive and tight.
I have used strut braces on a lot of cars, and some have made no feel able difference to vast difference. I have found on hatchbacks like golfs, a fully triangulated setup in the boot massively changes the stiffness of the rear, though a brace like that tends to take up a lot of boot space.
On a saloon Impreza a rear brace I feel is marginal if any difference, as you have the rear shelf and firewall area doing a lot of bracing. On the front it seems sti put them there so might be doing something.
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