Arguably the biggest advantage of the tally system is its universality. After all, the current numeral system inherited from the Romans is not used or understood by all people, especially those in Eastern cultures. Whereas countries in the Western world utilize numerical symbols like 1, 2 and 3 to signify values, these symbols are sometimes rendered obsolete when traveling to other places where, for example, Arabic or Chinese symbols are used. A tally insures global understanding of value.
The nature of many things that are counted increase in number over time. This is common during sporting events, for example. If statisticians are required to count the number of balls and strikes a pitcher throws or the number of steals and rebounds a basketball player gets, it is a far more practical for the statistician to keep a tally instead of repeatedly erasing values like 1, 2 or 3.
Educators will also find tallies are useful in teaching young children the basics of addition and subtraction. Using tallies to represent numerals can often aid in the visualization of the addition or subtraction process. For example, teachers can use materials like small wooden popsicle sticks to act as tallies so children can visualize and more clearly understand what numerals like 1 or 2 mean in a mathematical sense.
Of course, there are some disadvantages to using tallies, otherwise the Roman numeral system would have been replaced by tallies. The largest and most obvious disadvantage to using tallies in all cases is that tallies cannot easily express large numbers. Imagine a sign indicating that the distance to a nearby city was 120 miles -- clearly it is more efficient to write 120 than it is to write it in tally form.