Your utility power supply produces a sine-wave voltage waveform. Most lower cost UPS Systems will pass through the utility waveform when operating normally, but when in battery mode the inverter circuit produces a Square Wave.
This type of waveform is only suitable for switch mode power supplies as found in most computers, and devices with Switch Mode Power Supplies (SMPS). Modern computer PSU's now incorporate "power factor correction" circuits and these have been known to have issues with square wave inverters causing overloads and shutdowns.
The sine wave inverter circuit ensures you have maximum load compatibility, and the ability to run for extended periods of time without risk of degredation to your equipment.