What is the reason behind studying seroprevalence of a disease?
Seroprevalence tests people for the presence of a disease based on serologic methods. Often you do ELISA (or related methods) to test for pathogen specific antibodies.

This gives you information on how common a disease is, especially when it might infect people without them showing symptoms. It also can help detecting cross-protection when the seroprevalence is lower than expected. This is something shown in micronesia, where the seroprevalence was much lower than expected (from: Zika Virus Seroprevalence, French Polynesia, 2014–2015)


  Our findings show that &lt;50% of the population of French Polynesia had
  detectable Zika virus IgG. This seroprevalence rate is much lower than
  the 86% attack rate estimated by Kucharski et al. (14) using a model
  that assumed the French Polynesia population was 100% susceptible to
  Zika virus infection. However, in a setting where DENVs are highly
  prevalent (8), the possibility of cross-protecting immunity preventing
  infection from Zika virus (12,13) cannot be excluded. The attack rate
  and the asymptomatic:symptomatic ratio in French Polynesia were also
  lower than those described for the 2007 outbreak on Yap Island (73%
  and 4:1, respectively) (4); this finding supports the perception that
  the drivers of Zika virus transmission vary depending on geographic
  context. For other flaviviruses, such as DENV, previous model-based
  studies showed that the herd immunity threshold required to block
  viral transmission is ≈50%–85% (15). Thus, if Zika virus has the same
  epidemiologic characteristics as DENV, the seroprevalence rate of 49%
  would not be sufficient to prevent another outbreak.


It can also help to make predictions about the spreading of a disease. With a high seroprevalence epidemic spreading is unlikely, while this is pretty much different with a low prevalence. This is something which was observed in Brazil when Zika occured there first.