Your question is not perfect.Any way UPS not suply the the 
DC voltage is suply a/c only with out interupt.We use DC in 
control circuit because it has free from hormonics and it 
has constant.

Gangadhar say 
absolutly right

UPS is not DC it's AC....this equipment usually to support 
Control system such as Computer Sentral Control....this 
equipment is supplied by battery which the battery is DC 
source.....and convert by inverter inside UPS to AC source 
to supply the emergency equipment...
emergency equipmen is equipment where may not interrupt 
even the main source is blackout....
if you mean DC source this could be battery 
charger....battery charger ussualy used to operating 
controoler....such as PLC, also some electrical equipment 
is needed DC source...such as Circuit breaker, ACB(Air 
Circuit Breaker) motor inside the breaker is using DC source

dc voltage is necessary for l.t pannels was because dc is 
free from harmonics as well as smooth, in dc voltage  no 
variations are there so controlling becomes easier.
 no we can not use ups takes ac to charge battery store in 
dc and than when ever supply cut off than battery going to 
discharge voltage is converted into dc to ac one of the 
example in case of computer
 and also ups can be used for smaller duration so why we 
use UPS