while it's true that many people didn't do enough research outland vs inland and thought their only logical option is inland, for others inland has much more benefits; here r a few reasons why
1. some visa offices (outland) r actually slower than the inland process...
2. even if the visa office is not that slow but takes longer than 6 months there is a risk that the couple will be separated during the process - especially if the applicant had just a single entry visa and is NOT from a visa exempt country; some couples will do anything to avoid that especially if there r children involved too
3. with inland u will get a work permit after i think 10-11 months and for some people especially families with children it's very important
4. some applicants have no status in Canada and the inland application would or should give them an implied status
5. lastly, but maybe the most important some really cannot go to the home country if needed (for an interview for example) - maybe it's a war zone, maybe they risk going to prison, being tortured, God knows, this world is full of problems;
so really it depends from situation to situation; we have the fortunate nationalities vs less fortunate ones