To save something for a rainy day means to save money for no specific reason.

以备不时之需指的是不为特别的原因存钱。

In comparison, when you save something for a rainy day, you choose to put it safely away for future use.

相较而言，当你为了以防不时之需（save something for a rainy day），你会选择把它安全地放在一边以备将来使用。