In chapter 19, Bill Sikes explains that he wants a boy to help with the burglary of "that crib at Chertsey," and he says that the boy "mustn't be a big 'un." The implication is that Bill Sikes needs a boy small enough to fit through a panel on one of the doors or shutters to the house. Nancy then suggests that Oliver might be such a boy, also saying that "he's a safe one," meaning that he can be trusted.

Fagin agrees and pushes for Bill to take Oliver, in part so that Oliver can start "work[ing] for his bread." Fagin also says that all of the other boys are "too big" and that Oliver can be frightened into obedience. Bill agrees that Oliver is "just the size [he wants]" and assures Fagin that he shall indeed scare Oliver and kill him if he proves untrustworthy.

Fagin also thinks that it's a good idea for Oliver to be involved with the burglary so that he can start to "feel that he is one of us." Fagin says that, once Oliver has committed one crime, he shall be "Ours for his life," acknowledging that "it's quite enough for my power over him that he was in a robbery."

Fagin also says that Oliver is a useful boy to have in his gang because, with all the other boys, "Their looks convict 'em when they get into trouble." The implication is that Oliver is a good boy to have in the gang because he looks innocent and will, therefore, avoid detection or suspicion better than most.

Bill Sikes is initially somewhat reluctant to take Oliver along with him on his attempted burglary of the Maylie residence. The boy lacks experience in the ways of a criminal; and his lack of experience could easily lead to the gang's being rumbled by the police. But as Sikes has been casing the Maylies' place for two whole weeks without much success, he figures that it might be best to take Oliver along with him after all. Sikes reckons that the best way to get into the property will be through a small panel outside the house that can be lifted off, creating a little crawl space. Oliver is just the right size to crawl through that panel, enter the property, and then open the front door to let in Sikes and the rest of his gang of thieves.

In Oliver Twist, why did the thieves especially want Oliver to help with the burglary?

In the middle of Dickens' novel, the exploited Oliver Twist is taken by the brutal Sikes on a long trip outside London. Poor Oliver does not understand the purpose of this trip, but when they cross a bridge, the boy fears that he will be murdered and throw into the water. Finally, they arrive at a dilapidated house, where burglars wait for Sikes. When the infamous Toby Crackit sees Oliver, he asks who he is and Sikes replies that he is "the boy."

"Wud of Bister Fagind's lads [One of Mr. Fagin's boys]," exclaimed Barney, with a grin...."Wot an inwalable boy that'll make, for the old ladies' pockets in chapels!  His mug is a fortun' to him."

Crackit sees the good breeding and innocence upon Oliver's face as well as noticing Oliver's dimunitive size. His size is the reason why Oliver has been brought along, for the next day, Sikes takes him in the night to a house outside Chertsey.

And now...Oliver, well-nigh with grief and terror, saw that housebreaking and robbery, if not murder were the objects of the expedition. He clasped his hand together, and involutarily uttered a subdued exclamation of horror....
"Oh! for God's sake let me go!"

While the windows are barred, there is a small lattice window that is about five feet off the ground which has not been so barred. Because

[T]he aperture was so small, that the inmates had probably not thought it worth while to defend it more securely,

Sikes is able to release the latch and pass Oliver in through this window, giving him strict instructions how to reach the street door and open it. But, Oliver tries to mount the stairs and awaken the victims; seeing him Sikes calls out "Come back!" His voice alerts the "inmates" of the house, who fire pistols and Sikes returns the shots. Toby and Sikes run off with Oliver, who has been wounded. Later, Toby Crackit reports to Fagin that the robbery went badly and they left the wounded Oliver in a ditch. When poor Oliver regains consciousness, he drags himself back to the invaded house where the servants summon a doctor to tend the boy.

This episode, purposefully juxtaposed between chapters about the malevolent workhouse matron Mrs. Corney and the bullying almshouse beadle, Mr. Bumble, is yet another example of Charles Dickens's criticism of the Poor Laws of 1834 which demonstrated the callousness of many towards the plight of orphaned children.