The sight glass to enable you to see the oil is not necessarily essential, but not having it could cause you issues if you let the oil run out, as this can cause damage to your vacuum pump overall. It is very important to keep the oil levels topped up, so having a sight glass is a good way to simply and easily see how much is left at a glance.
Ultimately, while it is not “essential” it is definitely worth considering as a feature, and most of the best modern Vacuum Pumps out there do come with this as a way to monitor the oil levels within your pump.