Landsat 5 tm reflectance product images are delivered as signed 16 bit images, why is this preferred to unsigned 8 bit that the DN's are stored as level 1 products for the same satellite? Look at table 6-1 on page 19 of the following document:
In the case of Landsat 5 Surface Reflectance, the data is delivered as 16-bit signed integer to make the percentage-based nature of Surface Reflectance easier to understand. The whole point is that the number is easy to understand, relate to, and use modelling purposes.
As such, using 16-bit to store the information, even-though the L1 product is only 8-bit, makes the uptake of the data easier, at the slight cost of increased storage requirements and internet data usage. However, these two considerations are of minimal impact, as the data is tiny, compared to the other data amounts that the USGS deals with from other sensors.