Why are some human injections intraperitoneal?
IP injections are used for a variety of reasons:


The peritoneum provides a large surface area for absorption of drug (compared with intramuscular (IM) or subcutaneous); can thus inject a larger fluid volume.

Easier to inject that intravenous.

For some chemotherapy, such as ovarian cancer, IP injection is used to attempt to localize the drug near the tumor (it's through a catheter, rather than with a syringe and needle).  Essentially the goal is to provide chemotherapy to the abdominal region where cancer has spread rather than systemically.



 It is commonly done in animals, such as mice, because it is difficult to find a vein in small creatures.

https://en.wikipedia.org/wiki/Intraperitoneal_injection
http://www.cancer.org/cancer/ovariancancer/detailedguide/ovarian-cancer-treating-chemotherapy
http://www.ncbi.nlm.nih.gov/pubmed/20156151 courtesy of @AMR
http://cpharm.vetmed.vt.edu/VM8314/Administration.htm