It's basically an engineering dance to acheive certain goals for the aircraft (including marketing
)
But clearly either will work, since low-wing aircraft of the same general type (e.g. Piper PA-28 variants) very rarely use struts.
For the most part, Cessna deleted the strut in the 177 Cardinal (and probably 210) to:
- improve pilot sight-lines (no blind spot from the struts), also cleaning up the view for using the aircraft for aerial photography
- reduce drag, although that was more important in combination with retractable landing gear. Thereby improving fuel efficiency, range, or speed (at least in theory).
- iremove struts' obstruction to boarding or loading
- "look more futuristic" than the aging C172, which had been on the market for 15 years. Wing struts were something of a hold-over from the bi-plane era - looked a bit "old-fashioned" by 1970.
Cessna did, of course, have to make the 177's internal wing
spar stronger/stiffer/heavier, to compensate for the loss of the struts' added reinforcement.
In any case, the C172 is still made today, while the strutless Cessna prop planes have vanished from the marketplace. "The market" apparently seems to have had no problem with the strut.