What's the useful to place Cactus near computer?

Depends on the cactus. Ones with large spines are the best. Most computers put out electronic vibrational synchronicity waves EVSW that modify our thought patterns and cause all sorts of problems in our lives. The cactus spines absorb the waves and make our lives more serene and mellow. Try it and if you notice a change, put two cacti, one on each side of the computer. You will notice a big change and your life will get much better. More friends, promotion at work, that sort of thing.