It's a great app. You create a trip, add the dates, your daily or trip budget and then enter your expenses as you go, and break them into categories (eg accommodation, travel, sightseeing). Everything we spend is entered, from 1 Peruvian Sole for the toilet to 2.50 Bolivianos for bus departure tax. We can see how much we are spending via the graphs and it tells us our daily average.
You can even have several currencies in there at once.
Without it we would be scrambling around with notebooks and calculators. No need. Just enter what you spend add the category and it's all taken care of, you can even ad expenses in advance, for example if you are doing a tour or booked your accommodation in advance.
For example, our daily budget is £100 for the 2 of us, for all our expenses (except flights, inca trail and jungle safari, that is out of a separate budget) currently we are averaging £87.94. Under budget. It tells us how much we have spent to date in total and that we are 64% spent.