There is no point to using practices if they don't add any value. It's important to understand at least some of the benefits that can be obtained from using practices. Let's have a look at some of the common practices.
We can sometimes find good documentation. It describes the API or technology correctly and includes sample code. Sample code helps us understand the API as well as the concepts. I don't know about you, but I think in code; sample code is often easier for me to understand than prose, but , sample code is a double-edged sword.
One drawback of sample code is it may have the side-effects you're looking for, so you take it at face value and re-use it in your code. This is a form of pragmatic re-use.
Pragmatic re-use is when a developer re-uses code in a way which the original code was not intended to be re-used. This is quite common, and one of the most common forms of pragmatic re-use is copying and pasting code, such as copying and pasting the sample code as shown earlier.
In C#, classes are open for derivation unless they are modified with the sealed keyword to prevent inheritance. The lack of modification with sealed doesn't necessarily imply that the class is intended to be derived from. Deriving from a class like this is another form of pragmatic re-use because it's being re-used where re-use was not expected.
There are many motivators for pragmatic re-use. When a developer has neither the time nor the resources to learn code to perform a certain task, they often resort to a form of pragmatic re-use such as copy and paste.
Technical debt is a fairly well understood concept, but, it bears repeating as one of the potential motivators of best practices. Technical debt refers to the negative consequences of code, design, or architecture. There are all sorts of negative consequences that can occur from code. One common consequence is from code with no test coverage. The negative consequence of this is the lack of stability introduced from any change to that code.
Pragmatic re-use has the side-effect of taking on technical debt. At the very least, the code is doing something in a way it was never intended. This means it was not designed to do that, and therefore could never have been tested to work correctly in that scenario. The most common impetus for pragmatic re-use is that the developer either didn't understand how to do it himself, or didn't understand the original code. This means there is code in the code base that potentially no one understands. This means they don't understand why it works, how to test it correctly, what to do if something goes wrong with the code, or how to correctly change in response to changing requirements.
To be clear, technical debt isn't always bad. A team can take on technical debt for a variety of reasons. The important part is that they know the consequences and are willing to live with those consequences, maybe for a short period of time, to get some sort of benefit. This benefit could be time-to-market, proof-of-concept (maybe directly related to funding) meeting a deadline, budget, and so on.
There are all sorts of great sources of information on managing technical debt, so we won't get into technical debt beyond its impetus behind using best practices. If you're not clear on technical debt, I recommend as an exercise for the reader to learn more about it. Perhaps Martin Fowler's bliki (http://martinfowler.com/bliki/TechnicalDebt.html) or Steve McConnell's blog (http://blogs.construx.com/blogs/stevemcc/archive/2007/11/01/technical-debt-2.aspx) would be a good start.