You can cook or roast beef without searing the beef first.
But when you sear the beef before roasting it or cooking it you make the beef more tasty and give it a much better flavor.
You'll get the most flavor out of your beef when searing it first before roasting it because when you sear the beef you quickly cook the outer surface of the meat at a high temperatures so that it caramelizes and forms a crust.
I've cooked beef and even roasted beef without searing it before but whenever I sear the beef before roasting it or cooking it the beef turns out much better and tastes better.
That's how most restaurants cook their beef to make it have a good flavor.
They sear the beef first and then roast it or cook it other ways.