Walls can become wet because they are structurally compromised, where a building fault is allowing water to enter and pass through. It may be that there is a crack in some brickwork/render. Blocked gutters and plumbing issues can also result in penetrating damp. With water coming through to the inside of your property, you may see localised spots of black mould, paint blistering and damp patches - these are the common signs that you may be dealing with penetrating damp. In the UK, it is quite a common problem as we have a perfect storm of wet weather and old buildings that may have damaged outer walls. Tanking is a reliable way of dealing with the effects of penetrating damp once the root cause has been fixed.
Purpose Of Rendering A Wall
Rendering makes your wall more durable by making it water repellent. Lime in the mixture makes the wall 'breathable' thereby allowing moisture to escape. Moreover, its waterproof properties also protect your walls and floors from condensation and dampness which could result in mould and rotting.
The number one reason historically for rendering a property is to improve the look. Adding a render coat can really brighten up a shabby looking wall, and it gives the opportunity to give the whole house a facelift. If your render is old and tired, or perhaps cracked, it might be necessary to re-render also.