Lately I’ve been struck by some parallels between accent marks in Spanish and apostrophes in English.
First, accent marks can distinguish otherwise identical word pairs such as Hable ‘Speak!’ and Hablé ‘I spoke’, or tú ‘you’ and tu ‘your’. This is analogous to it’s versus its in English.
Second, they can help you pronounce a word correctly. For example, teléfono is pronounced on the third-to-last syllable (le), not the next-to-last as you would expect for a word that ends in a vowel (like cucaracha). Likewise, the apostrophe in English I’ll changes its pronunciation vis-a-vis ill.
Third, and perhaps most importantly, correct use of accent marks is a sign of an educated Spanish speaker. Even speakers who don’t bother to type accent marks in emails and text messages need to include them in more formal texts, such as school assignments and business letters. Omitting an accent mark is as offensive to many readers as is, for example, confusing they’re and their in English.