DELF and DALF are official French proficiency certificates issued under the French Ministry of Education. They belong to the same family and differ by level.
The levels
DELF covers levels A1 through B2; DALF covers C1 and C2. Each level is its own examination and tests all four skills: listening, reading, writing and speaking.
Once obtained, the certificate does not expire. That is unusual among language qualifications; TOEFL and IELTS results lose validity after two years.
When it is needed
Universities in France may require proof of French proficiency, usually at B2, and some programmes ask for C1.
There is an important exemption here. A student holding the French Baccalaureate is in most cases not required to submit a separate language certificate, because the diploma was obtained in French.
For a student at a francophone school
A student taking the bac usually has no reason to sit DELF or DALF. The diploma already demonstrates the language.
The cases where it does become useful are: leaving francophone education without taking the bac, applying to a French language institution outside France, or an institution explicitly asking for a certificate.
Where it does help
For a student who studied in French but does not sit the bac, this examination may be the only thing certifying their level. The fact that it does not expire makes it valuable in that situation.
Some scholarship and exchange programmes also ask for a certificate.
Making the decision
One question only: does the target institution accept the bac as proof of language proficiency. The answer is on that institution's own pages.
If it does, the exam is unnecessary. If it does not, or if the student will not take the bac, the level needed is identified and the exam is taken.