First, you may be eligible for premium tax credits or cost-sharing reductions (CSRs) to help lower the cost of coverage. You can only obtain those tax credits and cost-sharing reductions if you buy a plan through the health insurance Marketplace. There is no income limit on eligibility for premium tax credits, which cap your premium contribution at a percentage of your annual household income, so most people will do better to buy coverage through the health insurance Marketplace.
Second, if you use the health insurance Marketplace, you can compare plans and shop with confidence that all the plans displayed have been certified as meeting a minimum standard for coverage and quality.
Third, some special enrollment periods are only available to those currently enrolled in a Marketplace plan. For example, if you become newly eligible for CSRs, you are only eligible for a special enrollment period to enroll in a CSR-eligible plan if you already have Marketplace coverage. (45 C.F.R. §§ 155.420, 156.440.)