A target-date fund is a fund of funds with a schedule. In the early decades it holds close to 90% global equity. As the target year approaches it sells equity and buys bonds and short-term reserves along a published glide path, then keeps adjusting for a decade or two after the target date.
The appeal is behavioural as much as financial. Rebalancing happens without you, allocation drift never accumulates, and there is no decision to postpone.
The trade-off is precision. The glide path knows your retirement year, not your pension, your spouse's assets, or your risk tolerance. If those diverge sharply from average, a custom allocation may fit better.