Luke 8:12

‘And those by the wayside are those who have heard. Then comes the Devil, and takes away the word from their heart, that they may not believe and be saved.'

Those by the wayside were people who were like hard, beaten down ground, resistant and unreceptive to the seed of the word of God. And just as the birds had done, the Devil would swoop down and take the word from their hearts, lest they believe and be saved. He would not risk it lying there where it might be kicked on to good ground. As far as he was concerned God's seed had a nasty knack of sprouting where it ought not.

Jesus knew well from His earlier experience of temptation (Luke 3:4) the subtlety with which the Devil could come. And how he would soon plant thoughts which would remove the effect of a casual listening to the word of God.

If Jesus had not believed in a personal Devil there was no need to introduce him here. Some other interpretation would have been equally valid.