Why do we need resistors here? how to choose resistor to base pin? (and capacitor value recommendation)

So I built my first LED flasher using PNP transistors as per the circuit at the timestamp here: https://youtu.be/HrG98HJ3Z6w?t=56

As I understand it, when turned on the PNPs initially have nothing on the base so they pass between the collector and emitter pins turning the LEDs on. The flowing current also charges the capacitors which once charged, discharge into the base pins of the PNPs stopping the flow across the collector and emitter until discharged completely therefore causing flashing.

What I didnt understand was the need for the 100K resistors between the capacitors and ground. Why do they need to be there? My intuition says it is to make sure they discharge into the PNPs and not ground, is this correct? and how do we choose such values?

also more generally how does one go about choosing what resistor to put on the base pin of a transistor? I normally see a 1K resistor to the base of an NPN. Is this chosen from the data sheet? (I did have a look but don't yet understand enough to know what im reading)

On a related note, I currently have 5 x 100uF, 10 x 10uF, 10 x 100nF and 10 x 10nF capacitors, are there any other common values I should get? or should I just get a kit of electrolytics and a kit of ceramics for about £7 off ebay?

thanks for the help, its great to have a place like this when you are starting out on a hobby!