Hi feiutm9898,
In microcontroller there are thousands of transistors which are switching simulteniosly on high frequency due to which these type of spikes produces on I of VCC.It produces due to microconroller & may affect other circuit/peripherals of system.When you connect L(blocks AC & bypass DC) & C(blocks DC & bypass AC) combination as shown in figure 2-2 ,effect of spikes remain local to microcontroller while ripple on power plane gets reduce significantly.
Regards,
tramp