How to understand the role of testval, why this parameter greatly affects the model?

Questions
3

For example, in the algorithm for human deceit at the [Probabilistic-Programming-and-Bayesian-Methods-for-Hackers]

in pymc2, the code like this:

with model:
true_answers = pm.Bernoulli(“truths”, p, size=N)
first_coin_flips = pm.Bernoulli(“first_flips”, 0.5, size=N)
second_coin_flips = pm.Bernoulli(“second_flips”, 0.5, size=N)

in pymc3, the code like this:

with model:
true_answers = pm.Bernoulli(“truths”, p, shape=N, testval=np.random.binomial(1, 0.5, N))
first_coin_flips = pm.Bernoulli(“first_flips”, 0.5, shape=N, testval=np.random.binomial(1, 0.5, N))
second_coin_flips = pm.Bernoulli(“second_flips”, 0.5, shape=N,
testval=np.random.binomial(1,0.5, N))
why we set the testval in Pymc3