In addition to rice cookers, multi-cookers and egg boilers, there are devices specifically for cooking oatmeal, even though a rice cooker would do the job just fine. Perhaps the reason is that the minimal cooking time is shorter for oatmeal than for rice, so an oatmeal cooker speeds up the process compared to a non-programmable rice cooker. A programmable one could of course be set for the shorter cooking time.
The normal cycle length of rice cookers (30-45 min) and oatmeal cookers (20 min) is longer than the 12-15 minutes it takes to hard-boil eggs. However, overcooking a hard-boiled egg does not worsen its taste or texture significantly in my opinion, unlike for meat or vegetables. It turns out that oatmeal does not stick to eggshells that much, so eggs boiled in a cooker together with the oatmeal can be rinsed off quite easily.
Boiling the eggs and oatmeal together saves either time (if there is only one device available for cooking) or the labour of washing one more cooker (when using separate ones for eggs and oatmeal). Even with a dishwasher, disassembling a cooker, putting it in and taking it out of the washer and reassembling takes a few minutes. Rinsing the eggs is quicker.