Why do we need 'value', 'key', and 'query' in attention layer?

Question

I am learning basic ideas about the 'Transformer' Model. Based on the paper and tutorial I saw, the 'Attention layer' uses the neural network to get the 'value', the 'key', and the 'query'.

Here is the attention layer I learned from online.

class SelfAttention(nn.Module):
def __init__(self, embed_size, heads):
    super(SelfAttention, self).__init__()
    self.embed_size = embed_size
    self.heads = heads
    self.head_dim = embed_size // heads

    assert (self.head_dim * heads == embed_size),  "Embed size needs to be div by heads"

    self.values = nn.Linear(self.head_dim, self.head_dim, bias=False)
    self.keys = nn.Linear(self.head_dim, self.head_dim, bias=False)
    self.queries = nn.Linear(self.head_dim, self.head_dim, bias=False)
    self.fc_out = nn.Linear(heads*self.head_dim, embed_size)

def forward(self, values, keys, query, mask):
    N = query.shape[0]
    value_len, key_len, query_len = values.shape[1], keys.shape[1], query.shape[1]
    #split embedding into self.heads pieces
    values = values.reshape(N, value_len, self.heads, self.head_dim)
    keys = keys.reshape(N, key_len, self.heads, self.head_dim)
    queries = query.reshape(N, query_len, self.heads, self.head_dim)

    values = self.values(values)
    keys = self.keys(keys)
    queries = self.queries(queries)

    energy = torch.einsum("nqhd, nkhd->nhqk", [queries, keys])

    # queires shape (N, quesry_len, heads, heads_dim)
    # kyes shape : (N, key_len, heads, heads_dim)
    # energy shape: (N, heads, query_len, key_len)
    if mask is not None:
        energy = energy.masked_fill(mask == 0, float("-1e20"))
    attention = torch.softmax(energy / (self.embed_size ** (1/2)), dim=3)
    out = torch.einsum('nhql, nlhd->nqhd', [attention, values]).reshape(N, query_len, self.heads*self.head_dim)
    # attention shape: (N, heads, query_len, key_len)
    # value shpae: (N, value_len, heads, heads_dim)
    # after eisum (N, query_len, heads, head_dim) then flatten the last two dimmsions
    out = self.fc_out(out)
    return out

One thing I was very confused is why do we need the 'key', 'value', and 'query'? Could I just use only one of them? Or could I set more values besides these three? It looks like these three values are just transformed by three single-layer neural networks.