In addition to the many enhancements, and the ability to run native apps on the  Watch, watchOS 2.0 brings a lot of important security fixes. Even if you are not interested in its other features, this makes it an essential update.

Among others, Apple lists:

• Apple Pay (stops terminals from extracting some recent transaction information),
• memory corruption in the handling of audio files (could lead to a crash),
• updates to certificate trust policy,
• vulnerability in NSURL,
• issues in cookies,
• weak encryption of cache data (could allow someone with physical access to decrypt),
• cryptographic vulnerability which could reveal the RSA private key,
• bugs in handling font and text files,
• a bypass of code signing,
• bug in handling Disk Images,
• kernel memory corruption,
• ICU updated to 55.1,
• disclosure of kernel memory layout,
• SQLite updated to 3.8.10.2,

and more.