The use of right and down thrust is mostly a relic from the old free flight days. Without benefit of inflight control, a fairly predictable flight profile could result from angling the thrust line. Free flight models were launched with the engine screaming at full power and the idea was to gain the maximum altitude in the brief specified engine run. The airplane then needed to glide somewhere near its minimum sink.

With the airplane trimmed for its best glide, it would want to pitch up drastically at full power. Down thrust was used to control this tendency (along with a rearward cg location).

Since the purpose of the flight was to achieve maximum endurance and since straight line flight would quickly take it out of sight (and result in a very long walk to retrieve it), the airplane was trimmed to glide in a rather tight circle. A typical approach was to trim for a left turn when gliding and a right climbing turn under power. This required right thrust.

There were many variations on this adjustment but basically an angled thrust line was used to control the flight profile where no control input was possible after launch.

With a full range of inflight control and trim, thrust line offsets are relatively unimportant.