In context of geetar, certain model of geetar such as gibson sg/es335, fender jaguar/jazzmaster etc, the output jack is at the top of the geetar body(to know how it is like, use google picture to search for the pics of above mentioned model of geetars)
While a straight 1/4' phone plug can be used, for geetars as above, some prefer to use the angle plug so that you wont have an "over" protruding part from the guitar when you wanna rock out on stage. It can be for "cosmestic" issue as well, looking better(for some)
In context of effect pedal where you use those right angle plug, we have to see it in term of layout of pedals on a pedalboard.
pedalboard space can be precious. Each pedal enclosure are manufactured in different sizes and with the input/output jack situated at diffferent area on the enclosure(some jacks are along the "length" of the enclosure while some are placed at the "width")
in order to cut down on the space wasted on the pedalboard, using right angle plugs can allow the pedals to be placed closer to one another, resulting in having more pedals or simply neater and tider layout of the pedals without "protruding" straight plugs taking up the space on the board
sounds logical?