Generally, a usb-to-spdif converter is good for two scenarios:
1. you have a really bad usb source (like a computer or, especially, a laptop) where there is a not of noise, power supply is not good or the source usb transmit clock is not good. If you have a computer, you can get an excellent usb card with TCXO or OCXO click such as the Jcat or Pink Faun.
2. Your DAC has a very poor usb receiver board. This can mean the usb receiver chip architecure is poor or if the usb-to-i2s module doesn't have a good clock or power supply line.
If you have a good transport/source and a good DAC with a really good USB, then you generally don't need a usb-to-spdif converter. That being said, I generally like the sound of SPDIF, especially when I can use high end AES/EBU cables on source and dac. The engineers from Pink Faun (who make USB, SPDIF and I2S computer cards) have the opinion that usb audio does not sound as good as spdif audio. However, you need a really good SPDIF source and cable to reveal this.
In some situations, a usb-to-spdif converter with an extremely high end clock and power supply can be better than what you have. These are products such as the Innous Pheonix reclocker or the Audio-GD DI-20 audio interfaces. The Singxer SU-6 is okay, but there are some DACS that actually have a better usb-to-i2s module.