Dark Candi Sugar solves a problem plain cane or corn sugar can't: it adds real colour and a distinct rum, dried-fruit, and molasses flavour to a beer, while remaining almost fully fermentable — something no malt at a comparable colour can offer, since any malt dark enough to contribute that much colour also brings unfermentable body and roast or caramel character along with it.
At 300-500 °Lovibond, a modest addition darkens a beer substantially, and because it's produced through inverting and caramelising sugar rather than malting and kilning grain, it ferments out close to completely, leaving colour and flavour behind without the body a dark specialty malt at the same Lovibond would add. Belgian dubbel and quad are the styles most built around it — strong, dark, but deceptively light-bodied for their strength, a direct product of using candi sugar for depth instead of a heavier malt bill. Dark Candi Sugar isn't the same product as the crystallised "rock candy" sometimes sold under a similar name for other purposes; genuine brewing-grade dark candi sugar (or the liquid syrup version many suppliers now sell instead of the traditional solid rock form) is inverted and caramelised specifically for this fermentability-plus-colour combination. Special B or CaraAroma are the nearest malt-based alternatives for a similar flavour direction, though neither replicates candi sugar's essentially complete fermentability, which is exactly the trait a recipe reaching for candi sugar in the first place is usually built around.