A ‘plug-in’ or ‘add-on’ script is a bit of code that another developer has already written. It is normally written for a specific purpose and to be compatible with a specific CMS. A website designer doesn’t want to have to re-invent the wheel, so he will generally choose a CMS that has as many of these plug-ins available that will help him/her develop the site in the direction that you have instructed. This will save him/her development time and ultimately should save you, the client, money. Many development companies will push you towards what they know, rather than what is best for you - and you may become unstuck later down the line when their initial solution can't cope with your new requirement six months on.
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