let vs before blocks in rspec

gistfile1.rb

# Bad
describe '...' do
  before do
    @user = create(:user)
    @post = create(:post)
  end
  
  it '......' do
    expect(@user).to be_valid
  end
  
  it '......' do
    expect(@post).to be_valid
  end
end

#Godd
describe '...' do
  let(:user) {create(:user)}
  let(:post) {create(:post)}
  
  it '......' do
    expect(user).to be_valid
  end
  
  it '......' do
    expect(post).to be_valid
  end
end