It's more than just precision, it's making sure that the same value comes out that went in, and that things haven't been subtly altered via unintended conversions between decimal and binary floating-point representations. Obviously this is quite important when you've got both text and binary formats.

Some applications really need decimal values, and some really need IEEE floats. Ion can accurately and precisely denote both types of data, making it easier to ensure that the data is handled properly by both reader and writer.