what are the benefits of having the celta certificate tefl?

Besides teaching English to non-native speakers, I don't really see what else you can do. The benefits of such a certificate are to be seen only in terms of a career in TEFL; otherwise there's not much interest to it

Are you talking about TEFL or TOEFL (test of English as a foreign language)? If you mean the latter (which is a test, and if you pass it, you will be rewarded a certificate), this is one of the most valid and creditable standardised proficiency tests of English throughout the world. If you have its certificate, everyone (including employers) will make sure that you know English well enough.

CELTA and TEFL or TESOL certificates are on there own not too useful. However, they do open the door to a lot of teaching jobs if you do not have a degree. Say if you want to teach in Mexico or China, but you do not have any experience teaching or have a university degree, these certificates will help out in that field. Also, if you do have a degree, but not in education and you do want to teach ESL/EFL, it looks great on your resume. I did not need to have a TESOL for working in Korea, but because I had one, I got an extra $300 a month. The certificate paid for itself in just a few months.

Before you actually do get a certificate, try choosing where you want to go first and then deciding if it will help you get there.