Inconsistency causes confusion, because things don’t work the way the user expects them to. Forcing users to memorize exceptions to the rules increases the cognitive burden and causes resentment. Attention to consistency is important for instilling confidence in the product, because it gives the impression that there is a logical, rational, trustworthy designer behind the scenes.
For desktop and mobile applications, you should aim to understand and conform to the user interface guidelines for your operating system or platform. Consistency with these standard conventions reduces the number of new things a user needs to learn. Donald Norman