Waivers are especially common on new funds, where the expenses are a much higher percentage of assets early on. Those running the fund use waivers to build up the fund with a lower expense ratio, based on the assumption that with enough assets they can start charging something reasonable without a waiver.
Some funds also use it for advertising. When Fidelity lowered their expense ratio on some fund (forget which), they were actually using a waiver to do so. The fund's costs were higher than the expenses being charged, but they wanted to compete with Vanguard by pretending they had lower costs. The money has to come from somewhere - so someone else is paying for it. You can look on morningstar.com for a fund, and the "expenses" or "fees" page usually has the details.
