The beauty of cloches are they increase temperature, protect plants from adverse weather and some pests.
I use them in the string and again in the autumn, tending not to need them in full summer.
Spring they are used to warm the soil a bit faster and enable earlier planting. They also will keep frost at bay, not really long hard frosts though. Once things are happy and the threat of frosts is past I will remove them to a new site for whatever next I am going to sow.
Once we are into June they get put in the shed as they are not needed here in the south. This also helps extend their life as plastic is degraded by sun light. (Glass obviously is not effected)
Then come late September I start to sow winter salads for instance, once these are up and away I will protect with a cloche to extend the growing season into winter.
There are some problems with Cloches to consider, especially the plastic ones which can easily be damaged by wind and can be blown away if not adequately fixed to the ground.
Also they do get very warm in sun and need some ventilation in case this stresses the plants/seedlings. This usually involves small panels at each end of the tunnel that need removing on a morning if a sunny warm day is forecast.
Glass cloches are not so common now but if used the panes are fragile and can be dangerous if broken, may allotments do not allow glass on site for this reason.
Cloches come in various shapes and sizes. They can be round, domed or tunnel shaped and made out of plastic, glass, fleece, propanet, environmesh etc. They can be cheap but some can be very expensive. You can also make home made bell type cloches from mineral water bottles ranging in size from the normal 5 litres up to those you see in office water fountains, 50 litres??
All in all they are a great tool, flexible, mobile and effective.
Jerry