But, the solar panel will plug directly into your phone / headlight and charge it whilst you are on the move.
Charge your phone 1-day and the headlight the next, then repeat
In theory that works well. In practice, I found that the sunny conditions which were ideal for charging were just those in which I wanted to keep taking my phone out to take photos. Having it tucked away in the solar panel's charging pocket was a pain, and keeping it in my pocket meant leads trailing around me, and often seemed to end up with the plug falling out of the phone and it not charging anyway. Hence my conclusion that I needed a battery pack as well, allowing me to charge the battery pack during the day and the phone, from the battery pack, overnight. From which conclusion its only a short step to find I didn't really need the solar charging at all.
Don't get me wrong, I do think the solar panel has its uses. I still take it when weight is less of an issue - sailing or canoeing for example - and it lets me be a little more profligate with power use. But when backpacking I don't feel it provides enough benefit to justify its additional weight.