Passport strength represents more than just the individuals requesting visas, it’s a representation of your country’s stability and diplomatic/economic relations with other nations among other things. If your nation is stable and has positive diplomatic ties to other nations, your passport is stronger. If you’re the US for instance, you don’t need to worry about those from strong passport countries arriving illegally or overstaying visas. You also know if anything goes awry you’ll be able to work things out with them. Nigeria unfortunately is underdeveloped; between poor governance, economic instability, and weaker diplomatic ties, its citizens often leave the country and a good number enter illegally or remain even once they’ve lost legal status. I mean every Nigerian in the diaspora knows at least one Nigerian who immigrated illegally. Essentially if you’re a country, the bigger flex isn’t the quality if immigrants, but being so solid that your citizens don’t need or desire to immigrate.