Answer: These plants bloom on older wood, so an overall pruning will cause a gap in flowering. Pruning will also cause the plant to branch and stay dense over time. It is also used to control size, which can be important when you have to bring it indoors for the winter.
Typically they bloom less during the winter due to lower light levels indoors anyway, so some gardeners routinely prune them back in the fall when they come indoors. (This also helps if space is an issue.) During the winter the growth may be a bit leggy due to lack of light, so it may need some pruning again in late winter. Done this way, the plant may not begin blooming again until mid summer.
Alternatively, some gardeners will selectively prune year round so there is always a proportion of old wood, this can help the blooming stay relatively continuous -- if the plant has enough sunlight. It is a matter of personal preference how to go about it. I hope this helps.
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