Because if it didn’t, *Raul would be a single syllable.

Syllable rules

Two consecutive vowels in a word (h does not count) are pronounced as the same syllable when:

• It’s ui or iu. Example: ciu.dad, je.suí.ti.ca.
• a, e, o is next to i, u and i, u is not stressed. Example: can.ción, bue.no.

Two consecutive vowels in a word (h does not count) are pronounced as different syllables when:

• They are both the same vowel. Example: al.co.hol, le.er.
• a, e, o is next to a, e, o. Example: a.ho.ra, ca.ó.ti.co.
• a, e, o is next to i, u and i, u is stressed. These are always marked. Example: rí.o, ca.fe.í.na, Ra.úl.

Spelling rules

A word stress is marked when:

• It’s stressed in the last syllable and it ends with a letter that’s not a, e, i, o, u, n or s. Example: es.tá, in.glés, a.ten.ción.
• It’s stressed in the second-to-last syllable and ends with a letter other than a, e, i, o, u, n or s. Example: ár.bol, dé.bil, lá.tex.
• It’s stressed further up. Example: te.lé.fo.no, A.mé.ri.ca, o.cé.a.no.

Exceptions

• a, e, o is next to i, u and i, u is stressed, so you can tell they are different syllables.
• Words with only one syllable are not marked
• dé, él, más, mí, sé, sí, té, tú, qué, aún, quién, cómo, cuál, cuándo, cuánto, cuán, dónde y adónde are marked and have a completely different meaning from the unmarked words.
• The -mente suffix doesn’t count.

See https://www.rae.es/dpd/tilde for all the stuffy terminology and corner cases you never see in the real world.