Why do hamburger patties have ridges on one side?

What sort of streaking? A pattern of lumps or is there color streaking?

If there's color streaking and the meat is supposed to be raw, the meat's probably bad.

If there ridges or a pattern on one side it's from the hamburger patty press.

It does not matter which side you cook first. While with most meats you want to cook the "presentation side" (the side that will be up on a plate) first, with hamburger, because it's usually inside a bun and hidden, you don't generally cook one side differently than the other for any particular reason.

UPDATE:

The hamburger patty press uses dimples on one side (which produce the ridged pattern) in order to increase the pressure on the meat and the resulting compression of the patty. A more compressed patty results in more efficient packing (the same mass fits in a smaller volume) and a more stable patty (less likely to fall apart during cooking).

If you've ever seen roadwork which includes preparing a dirt surface to be layered with gravel or asphalt, there is a compression process involving what I think is called a sheep's foot roller, which is like a steam roller, except the roller/compression drum/wheel is studded with "feet" that result in a waffle-like appearance in the rolled dirt. This is done because this pattern increases the compression possible on any given portion of earth, creating a significantly more stable surface on which the road surface will be layed.

The hamburger patty press uses exactly the same technique for pretty much the same reason.

The main reason that your burger has these ridges on only one side is simply manufacturing. The ridges are put into the patties to prevent it from the middle coming up on you like bologna does sometimes when you fry it, and to also promote even heating and cooking. The only real reason most companies have for putting it only on one side, is that one flat side is the side they use to move the unfrozen patty around. There is a spatula type device that moves them around during production. If they slide something under the side with ridges before the patty is frozen it would mess it up before it was frozen. So you only see the pattern on one side on most of your mass produced frozen patties. Secondary reason is smaller but, some companies place pieces of wax paper between patties for easier separation after freezing, and the smooth side is the side that the patty rests on during or after freezing because it sticks to the wax paper more easily.

if you look closely at the Burger Press available here you will see the pattern that you are asking about built into the press. The lines are also in the bottom of the handle. The indentions and the dimple actual help promote even cooking through the meat. Also (according to the manufacturer) the dimple prevents the middle of the burger from swelling.