What might the reason for taking more than 1 Antenna into orbit / space?
It seems to be contested, but there is a consensus that more antennae increase the communication range of the craft. I believe the phrase Commnet is the term for KSP 1.2's new communication network simulation capability.
As far as I can determine, there is no decisive advantage in attaching more than one antenna to a craft in the stock (unmodded) game - it will not increase your transmission speed if you have multiple.
However there are a few cases where it may be desirable:
The most expensive antenna (the Communotron 88-88) is an electricity hog; if a latter stage of your rocket has reduced electrical generation or storage capacity (or you are planning for some accidental damage), it may be prudent to stick an additional cheaper antenna to your vessel so you can safely transmit without running out of power in these situations.
Spares in case a planned landing turns out less than ideal and some parts get destroyed on the process.
If your rocket is designed to launch probes, or otherwise detach a section to investigate science while the main craft does other things; each detachable section will need its own antenna, unless you plan to rendezvous and reattach to the main vessel.
You are OCD about symmetry, and for whatever reason it was not possible/practical to place the antenna on the center of your symmetrical axis. Also, I'll roll "Just for aesthetics" under this.
For stock KSP - there is no reason. One antenna is enough to transmit science.
Though there are mods that expand antenna usage, e.g. RemoteTech Also another option is to 'just stick-em on' to future proof your crafts for upcoming 1.1 update with enhancements regarding antenna usage, as mentioned in dev notes here.
Having more than one antenna allows you to upload more than one science result simultaneously. Your energy use will of course scale accordingly.