The problem with early glass cloches is ventilation. On sunny days, the heat can cause condensation in the cloche and damage the plant. Gardeners had to prop the cloche up with a small rock to allow air to circulate.
Modern cloches come with ventilation like a panel or window that opens. Square lantern-shaped cloches have detachable pyramid lids that can be rotated slightly, creating air holes.
They are a great cost-effective way to protect half-hardy plants without installing a large structure like a greenhouse. And they come in handy during spring and summer when you want to protect young plants from pests like slugs.
Cloches can also come in opaque terracotta, which is used in the spring to force rhubarb. It creates a dark, warm environment to encourage new shoots to grow.