All the answers so far miss the point. It's easier to cut across the grain as your saw needs only to break the fibers. Along the grain generates heat and a long curly fiber so you need a large-tooth saw and a means of getting rid of the heat. Ripping as it's called, is more difficult than a cross cut. But the real point about ripping wood from trees is that's how you get a strong plank or board. If you cross cut, then the resulting plank would be very weak and not a lot of pressure is needed to break a plank across the grain. Besides, a long plank can only come from a cut up the trunk. As for the grain or pattern, there are two main ways to cut a tree. You can look up those patterns on the internet, but to paraphrase here, one method produces a nice grain but more waste, and the other method is more economical. One of the ways tends to produce boards that can easily warp due to asymmetric shrinking caused by different radius cut through the growth rings one one side of the board compared to the other. For this reason, a cabinet maker will bond boards with alternate smiling and frowning grain to make a table top. The opposing pressures cancel each other out.
One time at least where you find cross cut is for a thick topped cutting board or bench for a butcher. The end grain of a hard close-grained wood resists cuts from a knife better than along the grain of a ripped plank.