This reply is for Norma responding to what good a phrase book is if you don't understand what is being said. (Ironically, I'm in Croatia as I type this, with my handy Croatian phrase book in my pocket.)
Lots of times, you aren't really looking for a reply--ordering in a restaurant or bar, for example. And plenty of times a "yes" or "no" is what you want in a response--like when you go into a shop and ask if they have what you need. So as long as you recognize a negative answer--and honestly, sometimes the tone alone carries the message--it was worth learning to ask for what you wanted.
Asking for directions usually ends up with a fair bit of pointing and pantomiming--but it does require that the asker start by asking a specific question. (I'm pretty directionally challenged, so one phrase I always try to learn is "Can you show me on this map?" Another handy one is "Can you write it down for me? I plan to use that tonight when I try out my Croatian "Can you recommend a good restaurant in this area?")
But I think the best reason to use a phrase book is that you are showing that you care about engaging with the culture of the place you're visiting. Order a glass of wine in Barcelona in English and you'll get the wine almost always. Order in Spanish, and you'll get a friendly smile and the wine. Order in Catalan, and you might get your wine comped, you'll be a sensation in the bar, and you'll have made a connection with the locals.
I do recommend that you spend a good bit of time working with your phrasebook before the trip, to familiarize yourself with as much of the sounds and structure of the language as possible. In my opinion, part of the fun of travel is preparing, and part of the fun of preparing is getting the rudiments of a language down.