Every language has regional varieties — different ways of speaking that vary by geography, social group, and context. Understanding dialects and accents matters because the language you learn needs to be consistent enough to sound natural.
Every person has an accent. Yes, that means you too. Maybe your accent is "standard," but it's still an accent.
Dialect vs. Accent
A dialect includes differences in vocabulary, grammar, and expressions. Mexican Spanish and Castilian Spanish are different dialects — they use some different words, phrases, and even verb forms.
An accent is specifically about pronunciation. Two people can speak the same dialect with slightly different accents.
But accent and dialect almost always come together. The distinction doesn't really matter and in the case of second language learning, the terms are interchangable.
Why You Should Choose
If you mix dialects randomly (using vocabulary from one region, pronunciation from another, and grammar from a third), you'll sound unnatural to native speakers. 
It probably won't prevent communication, but it's the equivalent of speaking English with British vocabulary, an Australian accent, and Southern US grammar — it's confusing and distracting.
Choosing a target dialect gives your brain a consistent model to learn from and sets you up for more natural-sounding speech.
How to Choose
- Pick the dialect of the country or region you have the most connection to (travel, friends, media)
- If you have no preference, pick the dialect most of your immersion content uses
- Don't overthink it. You can adjust later, and all dialects share the vast majority of the language
- If in doubt, the "standard" or "prestige" dialect of the language is a safe default (the scare quotes are to indicate our distaste of those terms, despite being widely used)
When to Decide
Phase 3B is dedicated to choosing a dialect and tuning your ears to it, but you may have already made an unconscious choice based on your immersion content.