I know nothing of ham radios. However, my elderly uncle, who lives next door to me has a ham radio. He has a very tall antenna. He gets a friend of his to string what looks like to me, an "air fence" around the property. He attaches what he calls a "loop antenna" to my house. This wire is strung through the trees. It doesn't create a complete circle. Is this really necessary?
The loop antenna is a different antenna than the primary antenna. Because the primary antenna and the loop antenna work differently, sometimes one antenna can be used to make contact with another station when the other antenna wouldn't work. Sometimes one antenna will work, sometimes the other antenna will work, and sometimes they both will work. So the loop antenna is probably not strictly necessary. Your uncle probably doesn't need to use both antennas to enjoy ham radio, but having two antennas is more fun for your uncle than having just one antenna.
It is most commonly unnecessary to need a loop antenna to make a (tall) vertical antenna work.
However, a horizontal loop antenna has a different polarization (like polaroid sunglasses for reflections) and pattern (roughly its aim) than a vertical antenna. And each antenna might be better at receiving signals that match their polarization and directional aim (if any). So having two antennas allows a HAM operator to switch between them and perhaps hear or send signals from or to a different set of far away (or nearby) radio stations.
To receive or transmit lower frequency signals, a loop antenna may need to be of a certain large enough size to work efficiently. Hundreds of feet for the lowest HAM band frequencies.
Also, power lines and stuff inside houses sometime radiate interfering electromagnetic waves. A horizontal loop might pick up less of this electrical noise, making it easier to hear more distant radio stations.