What is GNSS EPO orbit prediction in SIM868?
EPO is short for "extended prediction orbit". This document reveals the top level structure of the file. To my knowledge, there is no in-depth description that would allow you to craft such a file on your own (but I guess it should be easy to reverse-engineer). 

The almanac data embedded into the signal has only 154 bits (&lt;10 bytes) of carefully encoded data per space vehicle. In normal operations (GPS ground segment operable) one six day prediction for up to 32 satellites is broadcast every 12 minutes. The precision is limited to reduce the amount of data. (see IS-GPS-200H, Section 20.3.3.5.1.2 "Almanac Data", Table 20.IV).

The EPO data has 72 bytes of data per space vehicle, roughly seven times the amount of the almanac. I cannot tell how much of this is spent on multiple curve fits per satellite, on increased precision or simply on less efficient encoding.

Multiple curve fits for shorter time intervals, increased precision, correct time and known receiver position help to reduce the search effort for the signal after a loss of signal, reducing time to fix. Another advantage with EPO (and similar services from other vendors) is, that the full dataset for all satellites is available right from the start of the search. Broadcast almanac requires you to find at least one satellite unaided, then decode 12 minutes of signal to get the full set. 

For a tracker, it depends on how much wireless data transfer you want to accept (for downloading EPO from the internet). Refreshing EPO once in two days should be sufficient, more frequent updates will probably not shorten time to fix noticable. After download, store EPO in flash in addition to uploading to the module. After a power cycle, check timestamp of EPO and reuse it, if it's not too old.