Thyrwyn
Explorer
From the perspective of giving advice and for the purpose of general discussion, the concept of tiers already exists. We (the gaming community) talk about low level (characters, campaigns, adventures, ...) and high level (characters, campaigns, adventures, ...). Now, what each of those vague labels means may vary slightly from person to person, but the labels exist and are used. (Characters, campaigns, adventures, ...) which fall within those labels present their own challenges and capabilities.And Tiers are only redundant in the best case. In the worst case they make advancement even more rough by introducing huge changes at Tier borders but having minimal changes within.
Acknowledging general tiers facilitates conversation. They are defined by the general capabilities and challenges of the (characters, campaigns, adventures, ...) which they encompass. An 8th level (character, campaign, adventure, ...) has different capabilities and presents different challenges than a 12th level one, though most would consider both of those to be "mid-level". Without tiers, conversation either to has to default to discussing the attributes of each (character, campaign, adventure, ...) in terms of its specific level, or in terms of vague and potentially misleading generalizations.
And I am sure that Marketing would much rather be able to sell advice for each of 20 levels, than limit it to 4 tiers . . . .
Your mileage may vary.