Hello. If boost is already part of your workflow, there is probably no reason to switch. boost::program_options is sophisticated and widely used by the community.
If boost is not part of your workflow and you want something lightweight, then here are some reasons (some of these reasons are "in my opinion"): This library is header-only. It's a single header file and you're off. As the README suggests, the ArgumentParser class is easy to use and intuitive. Processing functions can be provided as "action" lambdas in each argument to convert the argument value from raw strings to higher-level objects and aggregates - examples in the README better represent what I mean here. The parser supports parsing and extraction of std::vector<T> and std::list<T> from a list of argument values too.
There are numerous argument parsers for C++ and this post should have been titled "Yet another argument parser for C++" but hey, I've had fun putting this together. If you think it suits your use-cases, you're welcome to try it out and give me feedback. Thanks.