The entire point of this guide is only to show the different starting options. Normally, I prefer cloth 5 start because you can play extremely aggressively early game to start snowballing, without many drawbacks. But this is only viable with an ADC you KNOW can preform well with you, especially coordination wise. Relic shield start is more for random ADCs, who you aren't as coordinated with and might lose some trades. The passive allows for sustain.

I'm not going to provide advice/tips on using his skills, it's expected that you become familiar with them. If you're just starting out with Thresh, I'd suggest you go the Relic shield build, with masteries and runes that allow you to be as tanky as possible.