Actually looking at my datasheet for a 555 it looks like it can source or sink 200mA so you should actually be able to drive a few LEDs off that easily. I was thinking it was lower, like 20mA or so, which could do one, but if you wanted more than that, it would burn up your chip. The transistor is used, because it doesn't draw as much current, turning it on and of, but can handle a lot more depending on the transistor used, so you can hook up a lot more LEDs to the transistor, and use the timer to turn it on and off, with out fear of burning out your timer chip. Depending on how many you plan on using, and of what type, you'll need to figure out the load created by the LEDs, and find out if the timer chip can handle it. But in your case, the PNP transistor would create the inversion you need, while also handling the current. Also don't forget the necessary resistors on the LEDs that will keep them from burning up also.