The most important and, perhaps, obvious answer to the above question is to communicate better. You will quickly learn (as with other languages) the same word may take on vastly different meanings with only a change in intonation.
For example, in Argentina, when the word “boludo” is pronounced as in “bolUUUUUUdo,” you can bet that there is some anger or frustration in the speaker’s sentence. However, a curt spitting out of “boludo” may just be a term of endearment used between friends.
People are also more comfortable speaking to someone in their own language. I know this sounds so obvious to the point of stupidity. Of course, an Italian speaker will be more comfortable speaking to you in Italian. Even so, people ignore this when it comes to Spanish. Speaking Spanish from Spain with someone from Chile will not create the same level of comfort as speaking Chilean Spanish to a Chilean.
There’s also the challenge involved in learning a culture enough to mimic it, or at least to fit in. It feels quite good when your Spanish is fluent enough that someone doesn’t recognize where you’re from,
or that you’re not a native speaker.
Another reason is that it is a good icebreaker when you want to meet people or make someone laugh. It’s always entertaining to hear a foreigner use typical slang phrases, words, and pronunciation in unexpected situations. Walk up to a Puerto Rican friend and say “¿Qué’ e’ la que hay?” Your friend will fall over laughing.
I recommend revising my 8 secrets to speak like a native covering practical tips on how to pick up the local Spanish.
Check out these other Learning Spanish Slang articles.