what's the purpose to crosslink receptors using Abs

[Takara]

In immunology literatures, I often noticed people using some antibodies to crosslink the corresponding receptors. I do not know the exact reason.
1. Why needs crosslinking using Abs? Does crosslinking make dimer or trimer of receptor formation?
2. What the signaling effects after crosslinking? active or block?
3. Which kind of antibodies can be used to crosslink receptors? Is it mentioned in Ab datasheet?
4. Immorbilized antibodies can be used to crosslink? How about soluble antibodies? Thanks a lot in advance