What is an expense tracker you ask? Why could I need it? Answer to the first question is quite easy: with expense trackers you keep your private and business expenses in one place, so you can always see how much money you have spent on what goods this month (week, year) and plan your budget accordingly.
Answer to the second question is not so easy, as there are millions people who just do not track their expenses and are doing fine. BUT: you can always improve yourself, and planning your expenses saves money, means makes you more rich, more predictable and your life more stable. So it may be a nice thing to use, actually.

The necessity to use expense tracker on cell phone or PDA is another thing to discuss. Compared to the PC applications, the mobile ones have usually less features and many restrictions because of the small screen and operating system drawbacks. Honestly, as I imagined typing in my thousands of expenses for the year 2006 into the 208x176 screen on my Nokia 6680, it was not a good feeling.
So the sense of the mobile expense trackers is that they are mobile. You are carrying them always with you. And that brings plenty of advantages that can not be achieved with PC software.
First of all, you can record the expense on the spot as you make it. So you will not spend one hour in the evening thinking on what you have forgotten. You can track your expenses without extra finding time for it ? like during stuck in the traffic jam or waiting at the reception.
Second, when you record one expense, it does not matter that the mobile is small: in order to record an expense you will need much less typing comparing with typing of an ordinary SMS.
Furthermore, you will not have to take extra records to a business trip for counting your expenses there, as your mobile you take anyway, and your boss will be happy having a nice report that all mobile applications deliver to PC. Usually such reports are already well formatted.
Finally, you really get a bit more organized and in total you spend less time for your budgeting and expenses compared with doing it on your weekends using PC.

After some considerations I have tried it. And I liked it. So now I can tell you how much money my insurance has cost me in last three months, and how much I have spent on entertainment last Sunday (too much, as always).

If you do not use expense trackers at all, you still can try it. And if you use only the PC applications there is always a chance to get also mobile ones, it will be really a new experience.