Why do people use these numbers and how safe is it?

Regarding this matter you're seeking help, more context is needed. You need to reveal what you need this temporary number for. Also, what country are you?

op oya use your hands and hold your two ears and drag them very well so you can pretend to hear the next words i am about to write below;

remember chidinma ojukwu!

you can leave your ears now

You don't open your personal data, and protect yourself from spam texts in the future. As for me, it is safer than to specify a real number on the Internet. Now a lot of such services, choose a reliable site, choose a number and get temp sms. This is enough for registration and very convenient. Only you should not use such numbers for personal accounts, as the number may be available not only to you.