THCA is the acidic precursor to THC, the most well known cannabinoid in the cannabis plant. The cannabis plant does not actually produce THC directly. Cannabis first produces CBGA which is converted by specific enzymes in the plant to THCA 

THCA is the acidic precursor to THC, the most well known cannabinoid in the cannabis plant. The cannabis plant does not actually produce THC directly. Cannabis first produces CBGA which is converted by specific enzymes in the plant to THCA  (among other cannabinoid acids). THCA then converts to THC through a process called decarboxylation.
This process can occur gradually through time but is sped up with the application of heat, with a temperature range of 110 to 130 degrees Celsius considered the optimum range for rapid decarboxylation (with in an hour).
In its acidic state THCA is not psychoactive and does not activate either the CB1 or CB2 receptors that are activated by THC.
Despite this THCA does have many established medicinal properties. It is a strong anti inflammatory and anti spasmodic and has demonstrated anti nausea effects akin to that of THC.
http://www.ncbi.nlm.nih.gov/pubmed/23889598
Though it is often credited as an anti cancer compound the evidence for this is far less established than other cannabinoids. Though other pathways may also be involved the primary receptors involved in the anti cancer effects of THCA are believed to be the receptors TRPM8 and TRPMA.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3570006/
http://www.ncbi.nlm.nih.gov/pubmed/24037916
http://jpet.aspetjournals.org/content/318/3/1375.long

THCA has shown protective properties of dopaminergic neurons that make it potentially very beneficial for sufferers of Parkinson's disease.
http://www.ncbi.nlm.nih.gov/pubmed/22571976

THCA has not been studied clinically for epilepsy but an increasing body of anecdotal reports has suggested it has tremendous efficacy for controlling seizures, with many patients reporting greater success with THCA extracts than with CBD dominant extracts, with some even reporting that THCA was effective where CBD was either less effective or ineffective. So while it lacks clinical confirmation, there does appear to be a lot of evidence to support the use of THCA as a treatment for epilepsy, especially in situations where CBD lacks efficacy.
http://www.ladybud.com/2014/04/02/cannabis-and-my-kid/
http://www.tokesignals.com/parents-thca-tincture-works-just-as-well-as-cbd-for-pediatric-seizures-heres-how-to-make-it/
http://www.medicaljane.com/2014/07/03/cannabis-classroom-the-role-of-cannabis-in-epilepsy-and-seizure-disorders/

Also not confirmed by clinical studies there are anecdotal claims that THCA is very effective for treating Lupus, with the most well known case reported by Dr William Courtney, who treated his wife Kristen successfully by juicing raw cannabis. Though Kristen now uses a combination of high CBDA and high THCA extracts, the initial success appeared involve only high THCA juices.
https://www.alchimiaweb.com/blogen/dr-courtneys-raw-cannabis-juice/

Unless decarboxylation is achieved, cannabinoid acids like THC do not activate the CB receptors associated with many of the therapeutic properties of cannabinoids. So decarboxylation is considered essential for maximising efficacy of therapeutic properties associated with the CB receptors. This is especially important when it comes to the anti cancer effects of cannabinoids like THC because THCA is unable to utilise the same receptors. However the anti cancer properties it has demonstrated may prove it to be of great value as a supplement to cancer treatments and perhaps even as a stand alone for certain cancer types.