What are the physical advantages of riding a single speed with higher chainrings? Ever since my aero road bike was stolen, I have been riding a cheap single  speed bike on average 6 miles a day. The crankset was cheap and I recently switched to Sram Ominium 48t vs what the bike had 44t. 

Overall the bike is faster but the chainring is on the larger side and i constantly have to standup if i am going up hill.

I feel like my lower back is getting stronger, but I am wondering what the physical advantages of a higher chainring on a single speed.
The efficiency of chain transmission decreases if one of the cogs has very low tooth count. Track cyclists often use high gear ratios, and large chainrings are used so that a reasonable sized cog can be used. Another reason is that when the cogs are larger, changing to one tooth larger or smaller cog changes gear ratio a smaller amount than with smaller cogs, so larger cogs allow finer tuning of the gear ratio. Both benefits are small, but in racing you use all advantages you can get.

For street singlespeed, the main reason to use large chainwheel is to make the bike look like track bike (isn't this why you bought the Omniums to begin with?). The other popular choice is 42t, because that comes standard with old road cranks and produces nice gear ratio for street use in combination with track cog.